Category: Health

New Year arrived with a bang, an explosion of coronavirus across Canada and the U.S. 

COVID-19 Data Explorer – Our World in Data

People reporting on TV, from politicians to public health scientists, seemed to be alarmed and a bit surprised. They shouldn’t be. Shortly after South Africa reported the new variant Omicron at the end of November, they provided statistics that showed that the number of cases was doubling every 2-3 days. This is really simple math. You don’t need a sophisticated epidemiological model, you don’t even need a calculator to see the impact of this over a few weeks. If 4 people with Omicron arrived in Canada from South Africa before the variant was identified, those 4 cases could have spread to 32,000 people by January 1, 2022, which is roughly what the latest case count is for Canada.

There were early reports from South Africa that Omicron is less severe than Delta. But the population profile in South Africa is a lot younger than the UK, Canada and the U.S. It was not so clear that this pattern of milder infections would be true for older people who have been a lot more vulnerable to the previous strains of the coronavirus.

On Dec 22, a careful study was published by Neil Ferguson and his team at the Imperial College London. He is one of the pre-eminent epidemiologists in the world. The analysis controlled for many variables such as age and sex and concluded that the reduction in the risk of hospitalisation for Omicron relative to Delta was 40-45%. This now seems to be an accepted characteristic of Omicron.
https://www.imperial.ac.uk/mrc-global-infectious-disease-analysis/covid-19/report-50-severity-omicron/

But even if the probability of hospitalization is lower, when there are so many more cases than Delta, there may be an overall increase in hospitalizations. In fact, patient admissions to hospitals have now started to increase sharply.

COVID-19 Data Explorer – Our World in Data

This is what is alarming many public health officials. This trend could overrun the health care system. So, many regions are re-introducing social controls. While most governments are insisting that “lockdowns” are a thing of the past, in fact what we are seeing now is “creeping lockdowns”.

One thing that does not seem to be reported very clearly is how long this wave will last. In South Africa it seems that it peaked quite quickly and is coming down (see first graph) but that does not mean it will happen here.

To try and answer this question, I updated my coronavirus model for the Omicron variant with the following assumptions:
1. By January 9, Omicron will account for 90% of the coronavirus cases in Canada.
2. Omicron is 4 times as transmissible as Delta.

Based on this, it does look like the Omicron wave will peak more quickly and decline more quickly than the earlier waves. The peak is towards the end of January with cases returning to pre-Omicron levels at the end of February. What determines when the decline begins is when the total immunity based on both vaccinations and natural immunity from people who have had Covid reaches a critical threshold. This model is very sensitive to the assumptions, so the peak cases may be more or less than 100,000 depending on how effective the various new restrictions are in reducing social interactions and transmission of the virus.

One thing that some epidemiologists are saying is that almost everyone will get Omicron, even vaccinated people. I don’t see anything like this. The total number of people in Canada who had Covid-19 at the end of the Omicron wave in the model, both symptomatic and asymptomatic, is about 8 million people out of a total population of 37 million. So about 30 million people will not get Covid because they are being protected by the vaccine, in spite of many breakthrough cases. So it does not make sense from a health perspective to surrender, throw caution to the wind and not worry about getting infected because “everyone will get it anyway”. It’s hard to predict if you will have a mild case or not. In addition, very little is known at this point what the effect of Omicron is on “long covid”. With earlier variants, 5% of people who were infected have lingering symptoms for many months and in some cases much longer.
What is my risk of getting Long Covid

The other thing some epidemiologists are saying is that this may be the last wave of the pandemic after which Covid will become endemic. It won’t disappear and will continue to infect people, but not at an epidemic rate, and will become more like a seasonal flu. However Dr. Fauci commented that most pandemics in the past have subsided after two waves, but Covid-19 has been unprecedented and has now proceeded to a fourth wave. So the most experienced experts in the field are really not able to predict what will happen. And the threat of another variant after Omicron is of course still possible.

The news has been full of reports of the Covid-19 Omicron ever since it was declared a variant of concern by the World Health Organization (WHO) on November 26, 2021. There has been a lot of new information every day and many changes to some of the earlier reports. The following is an excellent summary and assessment that is complete and up-to-date to December 18. Dr Andrew Morris is a professor of Infectious Diseases at the University of Toronto and a member of the Ontario COVID-19 Science Advisory Table. It is worth reading all of this and perhaps signing up for his regular mailings.

See Email#77 in Covid Emails

Here are some of the important highlights.

• The early data from South Africa suggested that Omicron might be less severe than Delta because hospitalizations from Omicron were less frequent than Delta but this is not being borne out in Denmark, and probably will not be true in Canada and the U.S.
• There are several other reasons why the progress of Omicron in South Africa may be very different than here. 
• Omicron is spreading rapidly around the world. Besides South Africa, UK, and Denmark, there are  steep Covid case increases in Norway, France and Spain.

What should you do?

Besides the recommendations made by Dr Morris in the article above, you should make sure you are using an effective mask. The typical surgical mask is only about 50% effective because it is not very tight fitting and there is a lot of space where virus particles in the air can get through to your mouth and nose. An N95 mask on the other hand is 95% effective. Cloth masks shouldn’t be used at all. 

One reason that Omicron may be so much more transmissible than Delta may be because it spreads more readily through the air. This could be because people who have Omicron may be expelling larger amounts of virus particles than with Delta or it could also be that the aerosol particles may be smaller, circulate more quickly and stay in the air longer. The evidence now suggests that Covid-19 is primarily spread through infectious aerosols that people breathe in.
Preliminary data hint at what makes Omicron spread so readily.

The previous blog article highlighted how ventilation is a big risk because there are no standards, from small spaces like restaurants to large spaces like factories and warehouses. Ventilation is very important for removing the small aerosol particles in the air. The larger droplets that contain the virus tend to drop out of the air within 6 feet shortly after being exhaled. Social distancing in enclosed spaces is not enough to keep you safe; the virus aerosol particles which may circulate in the air for hours may be more dangerous than the droplets.

It is difficult to assess how well a space is ventilated without some measurement or getting a statement about the HVAC system from the building management. And you cannot assume that a newer building will be better ventilated than an older building. 
Air Circulation and Coronavirus: How to Judge a Ventilation System

You can judge if a space is small relative to the number of people in it. So a crowded restaurant with low ceilings is probably not very safe whereas a gym with high ceilings that is not very crowded may be a lot safer. On the other hand, you could get infected in an empty restaurant from aerosol particles that are still circulating from a patron who left hours before you arrived. 

Finally remember that the best way to protect yourself from Covid infection is having several layers of defense. The following illustrates this well. 

At the end of September the 4th coronavirus wave peaked and began to decline. With increasing vaccination rates it was widely assumed that this was the last big wave of the pandemic in Canada and the US. However, in recent weeks case counts have plateaued and started moving up. Is this the start of a 5th wave?

Here is the projection for Ontario from the Covid-19 Science Advisory Table.

Here are the Covid cases for different regions in the U.S.

What happened?

One thing that happened is the emergence of new variants. The Delta Plus variant AY.4.2 may be slightly more transmissible than the Delta variant. It has made headway in the UK, outcompeting the Delta variant and now accounts for about 10% of sequenced virus samples there. To date it has not been widely reported in the U.S. or Canada.

Emergence of the ‘Delta Plus’ coronavirus variant

However, in Canada, some other Delta variants have been identified, AY.25 and AY.27. In Saskatchewan, AY.25 is becoming the predominant circulating strain, more than 50% of all cases. In Ontario, AY.25 accounted for 31% of confirmed cases.

What we know about Delta’s newest variants

This chart shows the spread of AY.25 and AY.27 in Saskatchewan.

In various parts of Europe there has been a sharp increase in Covid cases. Germany, which managed the pandemic better than most other countries (except Scandinavia) has more cases and hospital admissions than at any other time during the pandemic.

COVID-19 Data Explorer

The extent of the new Delta variants in Europe is not known. The most likely causes for the spike in cases are low vaccine uptake, waning immunity among people inoculated early and growing complacency about masks and distancing after governments relaxed curbs over the summer.

Why is Europe returning to the dark days of Covid?

What should be done?

Here is an assessment from a respected epidemiologist Dr. Colin Furness. He points out a number of “dumb” things that Ontario has done and how they should be corrected.

Allowing full stadium attendance was a sure way to allow further spread. This should be rolled back.

Restaurants are “one of the most dangerous places to be” because there are no regulations or standards for air filtration. The Ontario government has not officially acknowledged airborne transmission of the coronavirus. This will probably not be done by conservative governments in general because it would imply updating their health and safety standards to require many businesses from large warehouses to restaurants to incur the expense of upgrading their heating and cooling systems. However, requiring HEPA filters in spaces where large numbers of people are congregating is really fundamental to controlling the spread of the Delta virus and its variants. 

What is the likely outcome?

It was not possible to update the spreadsheet model because there are too many unknowns about the rate of spread and the effect of the new Delta variants. If these new variants do not spread more than 15% more rapidly than the Delta variant and if vaccinations continue to increase, especially now that the vaccine has been approved for children, my expectation is that in Canada and most regions of the U.S. the current increase in cases will be more of a bump than a 5th wave. However in low vaccination areas such as the American South and parts of Europe, the balance could be the reverse and they will experience a full 5th Wave.

I have noted numerous times how inept Public Health has been in handling the Covid-19 pandemic and the many mistakes that they made. But I was not able to explain in more than general terms why there were so many errors and what was wrong with the system. I have just finished reading an incredible book which explains a lot of this. It documents the many things that happened behind the scenes in the pandemic in the U.S. It tells the stories of many unsung heroes who really made a difference in overcoming the shortcomings of Public Health.

The book is The Premonition by Michael Lewis, published in May 2021. Here are a few of the surprising things that Lewis explained.

The bungled response in the US to the pandemic was not due to Donald Trump. There were many more failures in the system than all the outlandish statements and directives from Trump. Chief among them was the CDC. In many cases the CDC did not act when it should have and in some cases actually hindered some people in the public health system who were trying to contain epidemics, not just for Covid-19 but also earlier public health outbreaks.

The other surprising thing is that George W. Bush was the one who first developed a pandemic plan for the U.S. In 2005 someone gave him a book called The Great Influenza about the 1918 pandemic. There was a document from the Department of Health that laid out pandemic plans to speed up the production of vaccines and stockpile antiviral drugs. Bush said “This is bullshit. We need a whole society plan.” The disaster of 9/11 was still fresh in his mind and so he created a task force to create a comprehensive strategy for dealing with a pandemic.

The task force was interested in computational models that could predict pandemic spread. There were some academic models available but they were complicated, unwieldy and slow. Through an unusual sequence of events, they came across a usable model that had been developed by a 13 year old girl for a science fair and later refined by her father who was a researcher at Sandia National Labs.

Among all the incredible individuals and stories that Lewis uncovered, one person more than any other could be considered the hero. Charity Dean was a public health nurse working in California at the county level. She had a knack for seeing public health risks and had made some bold decisions to contain several infectious disease outbreaks, such as meningitis and tuberculosis. She was noticed and promoted to assistant director of the California Department of Public Health in 2018. 

When she saw the reports coming out of Wuhan in December and January 2020, she did some research and became very concerned. But when she tried to inform her boss, she was told not to use the word pandemic because it might alarm people. In the absence of direction from either the Whie House or the CDC,  she continued to try to press her case but she was barred from many meetings. Eventually, at the risk of being fired, she intruded on some meetings and her analysis and concerns eventually reached Governor Gavin Newsom. It was her interventions more than anything else that led California to issue a stay-at-home order in March 2020. This was the first state to take such action and it influenced many other states to do the same.

In this short excerpt from an interview, Lewis describes what he wanted to accomplish with this book.

In a more extensive interview Lewis describes the ‘ignored characters’ of the pandemic and why their premonitions were pushed aside.

Here is a more complete book review of The Premonition.

But I don’t think there is any substitute for reading the full book. Lewis is an incredible researcher and writer and there are many fascinating things that are not covered in any interview or reviews. Several previous books by Lewis have been made into blockbuster films.

Moneyball

The Big Short

On the back cover of The Premonition there is this quote from a book review.

I cannot imagine higher praise for an author than this. If Lewis wrote a history of the stapler, I would read it too.

The first blog article on the Delta variant, on June 18, 2021, showed an increase in cases in July but declining throughout August and beyond. This was more of a bump than a fourth wave.
https://anydaynow.blog/delta-variant/

The blog on July 19 revised the model projections by recalculating herd immunity for the delta variant when 80% of the population was fully vaccinated. Then vaccination rates were projected and herd immunity was predicted in August in Canada.
https://anydaynow.blog/herd-immunity-recalculated/

But here we are at the end of August and daily cases in Canada are still increasing.

What happened to herd immunity?

The big change was that at the beginning of August, the rate of vaccination decreased in Canada. On the current trajectory, 80% fully vaccinated will not be reached until October instead of August.

https://tinyurl.com/bn353kvy

This change results in a fourth wave that peaks with about 6,000 new cases per day at the beginning of October, 2021. After that, cases decline to a negligible amount.

It’s amazing how many people have became experts in public health in less than a year. Many of these people, like state governors and provincial premiers, use their newfound knowledge to make major decisions that affect the health of millions of people. Professionals working in the field of public health generally have at least six years of training, for a medical degree and/or a master’s degree in public health. Does anyone believe that politicians are so much smarter than health professionals that they can become experts in a year?

Public Health covers a very wide range of areas, from managing and monitoring disease to biosafety and security.
https://www.canada.ca/en/public-health.html

Contrary to what some people might think, public health did not just evolve in the last year with the onset of the coronavirus pandemic. Public Health has been an evolving scientific discipline for a long time.

Lockdown and other social distancing measures are not unique to the coronavirus pandemic. There are many articles and books about how plagues were dealt with in the past.

“From the onset of the Black Death in 1347-48, Italian cities which faced the Mediterranean, an epidemic sea, constructed a complex and articulated health defence system which was an example to all other western countries. The cornerstones of this health defence system lay in quarantine, sanitary cordons, lazarets, disinfection, and in the social regulation of the population at risk.”
https://pubmed.ncbi.nlm.nih.gov/19227587/

“Popular narratives continually included grievances about the cruelty and inequity of quarantine and the militaristic nature of its implementation. Despite these objections, quarantine remained a staple of the government response to plague outbreaks throughout the seventeenth century.”
https://pubmed.ncbi.nlm.nih.gov/22611587/

The Public Health Act in Britain was passed in 1848. It aimed to improve the sanitary condition of towns in England and Wales by placing the supply of water, sewerage, drainage, cleansing and paving under a single local body with the General Board of Health.

The Vaccination Act 1853 introduced compulsory smallpox vaccination in England and Wales.

The Infectious Disease (Notification) Act 1889 mandated the reporting of infectious diseases to the local sanitary authority, which could then pursue measures such as the removal of the patient to hospital and the disinfection of homes and properties.

The first public health organization in the U.S. was in New York City in 1866.

https://en.wikipedia.org/wiki/Public_health#After_the_18th_century

In 1905 the U.S. Supreme Court upheld the legitimacy of compulsory vaccination in Jacobson vs. Massachusetts. The Court compared the right to enact public health measures during an epidemic to the right of a government to defend its people from a military invasion. They compared the right to compel individuals to be vaccinated, whether they wanted to or not, to the power to conscript people to raise an army. This remains the major case in public health law today.
https://www.brandeis.edu/now/2020/may/smallpox-and-coronavirus-willrich.html

The current controversy raging over mandating vaccination for Covid-19 is at odds with long-standing principles encased in law going back over 100 years.

Why is there a debate over vaccine mandates? 

The various measures being proposed to mandate vaccinations are not compulsory orders requiring people to get vaccinated. They are just simple rules made for public health, that in specified areas, unvaccinated people pose a health risk to other vaccinated people in that space, and so unvaccinated people are to be restricted from entering that space. The space may be hospitals, restaurants, stores, or offices under the jurisdiction of a particular health authority. Such decisions are clearly within the bounds of public health authority.

Private companies also have the right to ban unvaccinated employees from the workplace for the safety of their employees. Many American companies, including those operating in Canada, require a medical exam as part of a job application. The information is not given to the employee but is used by the employer in any way they wish and may be used to deny employment.

One reason there is so much debate over vaccine mandates is that many public health officials have not taken initiative and leadership throughout the pandemic. Many have been working in a government bureaucracy for their whole career and they did not step forward when the pandemic was recognized to insist on instituting public health measures, even though that is supposed to be an important part of their job. They continually deferred to the political leaders, even when those leaders made bad public health decisions. A good example of this was David Williams, the chief medical officer of Ontario. He was a typical dry bureaucrat with little foresight or initiative and rarely challenged decisions made by Premier Doug Ford. In spite of criticisms and calls for him to resign, from many parts of the medical and public health community, Ford reappointed him for another term. However, shortly after starting his term, Williams finally saw the light and announced he would retire in June 2021.
https://www.cbc.ca/news/canada/toronto/ontario-top-doctor-pandemic-retirement-1.6046068

Only in extreme cases have public health officials overridden bad decisions made by politicians. One case of this was when Premier Ford in Ontario was very slow to bring in measures to control the large spike of the third wave. In response, the City of Toronto’s medical officer of health issued an order to close the schools, even though the Ontario Education Minister said the provincial government believed schools were safe and not sources of transmission of Covid-19.
https://www.cbc.ca/news/canada/toronto/toronto-closing-schools-covid-19-1.5976923

What should have been done?

If more public health officials had just done their job and issued orders to protect public health, without deliberating and knuckling under political pressure, a lot of bad decisions made in managing the pandemic could have been avoided. 

Restricting unvaccinated people from public places, such as hospitals, airplanes, trains, restaurants, shopping and office buildings where they are a health threat to others is just good public health practice, nothing more. The debate over requiring medical workers to be vaccinated is particularly ludicrous – people who go into a hospital for medical treatment should not be exposed to an increased risk of getting Covid-19. This has nothing to do with “personal freedom of choice”, it is fundamental public health where the health of the public is being protected. 

It is clear that a lot of people from citizens to people in power do not really understand what public health is and don’t feel it is an obligation and a priority of society, even though it is science that has been developed for hundreds of years and has corresponding laws that can be enacted by public health officials when necessary.

People who are demonstrating against vaccine mandates do not oppose the government requiring them to get a license to drive a motor vehicle. It is the law, and the purpose of the law is to protect innocent people from being injured by drivers who do not have adequate vision or safe driving skills. The purpose of public health is exactly the same – to protect innocent people from getting infected with a serious illness by people who are carriers of the disease.

In May, the blog showed that Covid-19 has a Herd Immunity threshold of 60%-75%. In other words, 60%-75% of the population must have immunity (from prior infection and vaccination) to prevent epidemic spread of the disease.

This was based on R₀, the basic reproduction number, being 2.5 for Covid-19 (green bar on this graph).

With the increase of the Alpha and Delta variants becoming the major coronaviruses in many countries, including the UK, Canada and the U.S., this has now changed.

The Alpha variant is 1.5 times more transmissible than the initial coronavirus.
The Delta  variant is 1.6 times more transmissible than the Alpha variant.

This increases R₀ to 2.5 x 1.5 x 1.6 = 6.0

Reading up from 6.0 on the horizontal axis of the graph we get a Herd Immunity threshold of .85. This means that 85% of the population needs to be immune. This is a big part of the explanation for the sudden spike in cases in the UK and the U.S.

https://tinyurl.com/76t5h6vn

The other part of the explanation is that while single vaccination rates went up sufficiently to protect against the Alpha variant, protection against the Delta variant really requires full vaccination. The immunity level of fully vaccinated people has not reached the 85% level in any of these countries. Add 10% to the numbers in the following graph to include people with immunity from prior Covid-19 infections to get the total immunity level for the population.

https://tinyurl.com/bn353kvy

The other big problem is that as the number of people vaccinated increases, there is a slower vaccination rate. People with vaccine hesitancy make up a larger proportion of the people who are unvaccinated. The graph above shows this as a decrease in the slope of the line. This has occurred in the UK and very significantly in the U.S. since mid-May. Canada does not show this slowdown yet.

A simple projection of the above chart, assuming these countries maintain their current vaccination rate, shows the following results for reaching herd immunity when about 80% of the population is fully vaccinated:

Canada in August
UK in November
U.S. in January 2022

Covid-Zero is a strategy to impose a range of restrictions including strict lockdowns and rigorous testing and tracing to drive transmission of Covid-19 to zero. Several organizations have been set up to promote this strategy to end the coronavirus around the world.
EndCoronavirus.org

It has had remarkable success in a number of countries, such as Australia, Vietnam, Taiwan and New Zealand.
Get Real Canada, Get to Zero

However, Australia’s success with Covid-Zero seems to have run into a roadblock. It seems like the Delta variant spreads too quickly for this strategy to continue working. Who would have predicted this a few months ago?
Why the Delta Variant Could End Australia’s Pursuit of ‘Covid Zero’

Many countries in Asia who have also done really well keeping Covid cases low are running into the same turn of events.
As Delta Variant Surges, Outbreaks Return in Many Parts of the World

As vaccinations increase in Canada and Covid-19 cases continue to decline, there is a general sense of optimism that the pandemic is almost over. However, reports from the UK on the rapid spread of the Delta variant, first discovered in India, provide an ominous forecast.

In the past, trajectories of the coronavirus in Europe and the UK have not been followed very seriously in North America. Our “experts” and political leaders have adopted a “wait and see” approach. This happened with the original coronavirus outbreak in March 2020, the second wave that started in September and the third wave (due to the Alpha, or UK variant) in March 2021. The result was that in most cases public health action was delayed and not optimal.

In a crisis, this is not the right thing to do. The right thing to do is to be prepared and plan for the worst. 

Here is what has been reported in the UK (June 11).
– 90% of new Covid cases are now the Delta variant.
– Cases are doubling about every 10 days.
– The Delta variant is 60% more transmissible than the Alpha variant, which makes it 140% more transmissible than the original Covid-19.
– The first vaccination is only about 30% effective; but the second shot provides 80% effectiveness.
– There are some cases of fully vaccinated people getting Covid, and dying.
https://www.theguardian.com/world/2021/jun/11/delta-variant-is-linked-to-90-of-covid-cases-in-uk

Modelling in the UK shows that a third wave of infections could rival Britain’s second wave from the Alpha variant. To counteract this, Prime Minister Boris Johnson delayed the final stage of easing lockdown restrictions for 4 weeks until 19 July.
https://www.bbc.com/news/uk-57476776

What’s the risk in Canada?

The last projection in the May 12 blog predicted “restrictions being removed in July. There is no surge and cases continue to decline to a negligible number”. 

But this did not include the Delta variant. It has now been added to the model using the characteristics of the Delta variant summarized in the following report, assuming 16% of all Covid cases are the Delta variant on June 18, rising to 90% in the first week of August.

In addition, the relaxation of controls was applied, using the guidelines that most provinces are following. These are the steps in the Ontario reopening plan, with at least 21 days between steps.

Step 1: 60% of adults vaccinated with one dose.
Step 2: 70% of adults vaccinated with one dose and 20% vaccinated with two doses.
Step 3: 70 to 80% of adults vaccinated with one dose and 25% vaccinated with two doses.
https://www.ontario.ca/page/reopening-ontario

Ontario, B.C. and Alberta entered Step 1 in the second week of June.
The model projects proceeding with
– Step 2 in the first week of July.
– Step 3 in the last week of July.
– Fully reopen first week of September.

Here is the picture with these assumptions.

So cases continue a rapid decrease into July but start to rise as controls are relaxed and the Delta variant spreads.

It remains to be seen which provinces delay advancing to the next reopening step when cases increase or just “wait and see”, allowing the Delta variant to spread and hoping that the increase in full vaccinations will reduce the spread in September.

What’s the risk in the U.S.?

The fully vaccinated rate is 44% In the U.S. compared to 14% in Canada. This means there is more resistance to the spread of the Delta variant. But all states except 4 have reopened completely which allows the coronavirus to spread more easily.
https://www.nytimes.com/interactive/2020/us/states-reopen-map-coronavirus.html

The U.S. could see a fourth wave similar to the one in Canada in the summer or autumn. There are also many regions in the U.S. that have very low vaccination rates. There will likely be several outbreaks in these parts of the country as the Delta variant spreads. 

How to keep up with the Delta variant?

You can easily follow the Covid-19 trend without waiting for a news report. Just watch this chart “New Covid-19 Cases per Million People” which is updated daily.
https://tinyurl.com/76t5h6vn

It clearly shows the Delta variant in the UK taking off on May 25. If the curve for Canada or the U.S. starts a steep rise like this, you know they are headed for trouble.

Herd Immunity occurs when a sufficient percentage of a population has become immune to an infection, whether through vaccination or previous infections, thereby reducing the likelihood of infection for individuals who lack immunity. It is the point where the disease reaches an endemic steady state, which means that the infection level is neither growing nor declining exponentially.

At the beginning of the pandemic it was not clear if Covid-19 was like other viral infections and if people would acquire immunity after recovering from it. It was recently confirmed that this is in fact the case.
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)00675-9/fulltext

Here is a chart of Herd Immunity  for different diseases. Note that Covid-19 has a Herd Immunity level of 60%-75%.
https://en.wikipedia.org/wiki/Herd_immunity

R₀ the basic reproduction number is the average number of people that get infected by someone who has the disease. R₀ is basically a measure of contagiousness. It was measured early in the pandemic as 2.5 for Covid-19.

The Herd Immunity Threshold (HIT) is different for different diseases. It is calculated as  1 – 1/R₀

---

In December 2020, Dr Fauci stated that the real range to reach Herd Immunity is when 70%-90% of the population has immunity, instead of 60%-70%.
https://www.nytimes.com/2020/12/24/health/herd-immunity-covid-coronavirus.html

More recently the thinking has shifted to saying that reaching Herd Immunity Is unlikely in the U.S.
https://www.nytimes.com/2021/05/03/health/covid-herd-immunity-vaccine.html

What’s going on here?

It seems like some people are now using a different definition for Herd Immunity. Instead of “reducing the likelihood of infection” it is now being used to signify the point at which the virus disappears.

If we use the classic definition, latest statistics give a pretty good indication that the U.S. has achieved Herd Immunity. The new cases (and hospitalizations and deaths) have been in decline since April 15, down by 33% in three weeks. If there are no further surges and social restrictions can be removed, then this is Herd Immunity.

Note that for the first time in the pandemic, the new case rate in Canada has exceeded the U.S. Cases in Canada have flattened but not significantly declined. The difference is that the U.S. has had a much higher vaccination rate, and also a higher number of prior infections.

https://ourworldindata.org/explorers/coronavirus-data-explorer?zoomToSelection=true&time=2020-03-01..latest&pickerSort=asc&pickerMetric=location&Metric=Confirmed+cases&Interval=7-day+rolling+average&Relative+to+Population=true&Align+outbreaks=false&country=CAN~USA

The level of immunity in the U.S. on April 15, 2021 was 48%.
  (38% from vaccination plus 10% from people who had recovered from Covid).

In Canada, immunity was 25%.
  (22% from vaccinations plus 3% from people who had recovered from Covid).

https://ourworldindata.org/explorers/coronavirus-data-explorer?zoomToSelection=true&time=2020-03-01..latest&pickerSort=asc&pickerMetric=location&Metric=People+vaccinated&Interval=7-day+rolling+average&Relative+to+Population=true&Align+outbreaks=false&country=CAN~USA

Several months ago, the spreadsheet model I have been using since the beginning of the pandemic, projected herd immunity in the U.S. would occur in April 2021 with an immunity level around 50%, matching these actual statistics. This verifies that the basic logic in the model is in the right ballpark. 

To gauge what will happen going forward, these adjustments were made.

1 . To reflect the vaccine hesitancy that is occurring in the U.S. the average daily vaccination rate was reduced. The maximum number of people that get vaccinated was set to 70% of the population. 

2. To reflect the removal of pandemic controls, the reproduction number was increased to the Covid-19 basic number of 2.5 in mid-May.

3. The UK variant was set to be 90% of new cases.

The result shows the U.S. reaching 75% immunity in mid-May, based on 60% of the population being vaccinated with at least one dose. There is no surge and cases continue to decline to a negligible level. This is Herd Immunity corresponding to the more strict definition.

Since this is a national model, it does not reflect areas of the country where the vaccination rate is much lower than the national average, such as Alabama and Mississippi, where Covid outbreaks may continue to occur.

---

The forecast for the Canada model predicts cases declining in May. At the height of the third wave, this may not look likely, with major outbreaks still underway in Alberta, Manitoba and Nova Scotia. But because cases have been coming down in the large provinces (Quebec, Ontario, B.C.), the national average shows this trend, which will occur later in the smaller provinces.

The model predicts restrictions being removed in July when Canada reaches 74% immunity and vaccinations reach the 70% limit. There is no surge and cases continue to decline to a negligible number. This is Herd Immunity corresponding to the more strict definition.