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Prior Infection Gives Far Better Immunity Than Vaccination Against Both Delta and Omicron Strains

In my previous blog post I analyzed the data from the British government disclosed in Ferguson et al., along with South African data in Maslo et al., to show that the Omicron variant is far less deadly—like 38-fold less deadly!—than the starting Alpha and Beta strains of COVID. (The Delta variant incidentally is intermediate, at about a third less deadly than the Alpha and Beta strains.) I realized that the Ferguson paper also gives data on the prior infection status and vaccination status of the persons diagnosed with either Delta or Omicron infection in the same time period of Nov. 12 to 29, 2021. That allows me to calculate the efficacy of the vaccines and of prior infection in preventing infection! Here are the numbers from Ferguson et al.


Table 1 in the Cases column shows the number of diagnosed cases with the Delta variant or the Omicron variant in the United Kingdom from Nov. 12 to 29, 2021, by vaccination status and by status of prior infection with COVID. The second column is the number of people in the U.K. divided by 100, based on 80% of the population having had 2 or more doses of the vaccine (reference 2) and 60% of the population having been already infected with COVID by Nov. 12, 2021 (my estimate). The third column is then the percentage of each population group that were infected with Delta or Omicron in the Nov. 12 to 29 time period. The fourth is the relative risk of infection of the unvaccinated versus vaccinated and of the not previously infected versus the previously infected.


What this shows is that for the Delta strain, vaccination cuts risk of infection about 3-fold or 66% protection. In contrast, prior infection cuts risk by 99.2%! It virtually eliminates your risk of even being infected with Delta, let alone dying of it. Prior infection gives far better protection than vaccination.


For the Omicron strain, by these numbers vaccination only reduces risk of infection by 7%. It has almost no efficacy. But prior infection cuts risk by 93%. It is extremely effective.


By these numbers, vaccination is almost useless against Omicron. But Omicron almost never causes death (prior blog post) so there is no reason to vaccinate against it anyways. And prior infection with any strain is quite effective in reducing infection with Omicron and is certainly even more effective in reducing the already miniscule risk of death or serious illness with Omicron.


If these numbers from Ferguson, the U.K. government, Maslo, and the South African government (analyzed in my prior blog post) are correct or close to correct, then there is no reason to vaccinate any more once Omicron is the predominant strain. There is no reason to vaccinate because:

  1. Omicron almost never causes death and rarely causes serious illness, even in unvaccinated people. It is about as deadly and dangerous as the common cold.

  2. The existing vaccines are almost worthless against Omicron (this blog post).

  3. By next summer, over 80% of the population will have been infected with COVID and will have far better immunity against Omicron and any new strains that evolve than any vaccine could give.

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