We will be at herd immunity when the Omicron wave subsides by about the end of February.
In prior posts I calculated that prior infection gives 99% prevention of new infections with Delta and 93% with Omicron, while vaccination gives only 66% prevention of new infection with Delta and only 7% with Omicron. Therefore we cannot get to herd immunity with vaccination. Vaccination prevents death, but for the most part not infection.
R is the number of persons each infected person infects in turn and R0 is the special case of R in a naïve population where no one has immunity at the beginning of an outbreak. R0 for COVID-19 was estimated at 3 to 5. As the epidemic continues, R drops from R0. If R0 is 3, in principle when 2/3 of the population have been infected and are therefore immune, R has dropped by 2/3 and is now only 1. When R drops below 1, it means the average infected person infects fewer than one new person and therefore the epidemic dies. It is not that you get no new cases, but they drop off. So, at worst, we should be at herd immunity with R at 1 or less by the time 80% or 4/5 of the population is infected, even if R0 was 5.
This table shows what percentage of the U.S. population I estimate had been infected by these four dates.
For Dec. 1, 2020, and May 1, 2021, I calculated the number of infected persons by dividing deaths by 0.0040, based on my calculation that the infection fatality rate or percentage of infected people who die was 0.40%. I calculated it was 0.46% at the start of the epidemic and somewhat less later as the virus evolved to be less deadly and we got better at treating infection, so I figured on average it was about 0.40% in unvaccinated persons until the vaccines came along.
For the bottom row, I calculated that since there were 18.9 million new recorded cases between Dec. 1, 2020 and May 1, 2021, and 23.6% of the population became infected in that time by my estimates, each million new recorded cases represents infection of 1.2% of the population (23.6% divided by 18.9 million) = 1.2% per million). From the increase in officially recorded cases from May 1, 2021 to Dec. 1, 2021 and January 31, 2022, I calculated the increased percentage infected and added that to the 45% infected by May 1, 2021. I switched the method of calculation from deaths to recorded cases after May 1, 2021 because that was about the date by which most susceptible people had been infected and therefore the infection fatality rate should drop and deaths should underestimate how many people are infected.
From these numbers, 67% of the population was infected by Dec. 1, 2021, and 97% now on January 31, 2022. I don’t believe we actually have had 97% infected now. But 65% by Dec. 1, 2020, I think was probably about right. I have confidence in the infection fatality rate of 0.40% in an unvaccinated population. Most researchers calculate a lower number than that and nobody thinks it is above 0.65% or so. If anything that number is probably high, and it certainly should be high once most of the population is vaccinated, as it has been since about May 1, 2021. Just using deaths, with an IFR of 0.40% we had 62% of the population infected by Dec. 1, 2021, which certainly should be a low estimate.
The 97% number as of January 31, 2022, has to be too high. It would mean that 9 of 10 people who were not previously infected have been infected in the last two months.
Those two months correspond to Omicron. I have calculated elsewhere that prior infection gives 90% protection against Omicron. If so, then only about 2/3 of the new cases since then would be in previously uninfected people and 1/3 would be in the 67% of the population that had been infected by Dec. 1, 2021. So in that case, instead of 30% of the population being newly infected, only 20% would be and we would be at 85% having been infected now, not 97%.
But 85% should get us to herd immunity. Once the Omicron wave dies down in the next month, we should have few cases going forward forever. Omicron laughs at vaccination. Vaccination only gives 7% protection against it. But prior infection still gives over 90% protection. Going forward, new strains will evolve from Omicron that continue to completely evade immunity from vaccination and do a decent job of evading immunity from prior infection. But, like Omicron, they will cause only mild illness—much less severe than ordinary influenza and not much more severe than the common cold. Every year, we will continue to get cases of this. It will be endemic and permanent, like the common cold, but cause no more deaths than influenza, both because it is inherently non-lethal and because the large majority has prior infection with a related strain that gives partial protection against infection and pretty good protection against severe disease.
So we are basically done with this. It will continue every year at a level of infections and illness similar to the common cold or at worst approaching the flu. I will predict, however, that the powers that be will continue to cling to this as a crisis that justifies their control over our lives.