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.
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).
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.
