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Omicron and Vaccine Effectiveness

On December 8, CDC Director Rochelle Walensky said 43 Americans in 19 states had been infected with Omicron. Thirty-four of the patients had been fully vaccinated, 14 had been boosted, and 6 had previous COVID19 infections. One fully vaccinated patient required hospitalization. Twenty-five patients (58%) were 18 to 39 years old and only 14 (33%) reported international travel during the 2 weeks prior to symptom onset.

Epidemiological studies in South Africa and England found that many people infected with the Omicron variant had already been infected by earlier SARS-CoV-2 variants. The risk of reinfection with Omicron has been estimated to be about five times greater than for other variants. The ability to evade either vaccine or natural immunity appears to be one reason why Omicron cases are increasing so rapidly.

Two recent studies provided some early clues about the ability of Omicron to evade neutralizing antibodies induced by vaccines or previous infection. These were in vitro studies testing the ability of small numbers of patient’s blood samples to neutralize the Omicron variant compared to either the original SARS-CoV-2 virus or the Delta variant. A study from South Africa found a 40-fold reduction in antibody neutralization, both in people who received two doses of the Pfizer-BioNTech vaccine as well as people with hybrid immunity from both natural infection and a dose of Pfizer vaccine.

A Swedish study found a sevenfold reduction in blood samples from random donors and a fivefold reduction in samples from those who had earlier COVID-19 infections. A third study from the University Hospital Frankfurt reported a 37-fold reduction in neutralization against Omicron in the blood of people who had received three doses of Pfizer, when compared with Delta. She also found no measurable Omicron neutralization 6 months after two-dose regimens of Pfizer, Moderna, or a mix of Pfizer and AstraZeneca.

Soumya Swaminathan, MD, the WHO's chief scientist, concluded that booster doses with the current vaccine are probably not the solution, though officials are still looking at the evidence.

However, it must be remembered that vaccines not only stimulate our immune system to produce neutralizing antibodies but also induce the production of memory B cells and T cells. Even though the concentration of circulating neutralizing antibodies wanes over time, T cells and B cells are rapidly mobilized after exposure to SARS-CoV-2 to destroy viral infected cells and synthesize additional neutralizing antibodies. This explains why in vitro studies demonstrating declining levels of neutralizing antibody may be associated with an increased risk of reinfection, but not necessarily indicative of an increased risk of severe disease, hospitalization, or mortality.

In the early days of the Delta variant, it was found to evade vaccine induced neutralizing antibodies more than previous variants. However, the risk of serious outcomes has remained largely unchanged for vaccinated people with intact immune systems. If Omicron behaves similarly, it may cause:

  • More infections among the unvaccinated with similar rates of severe illness
  • More reinfections among people previously infected with other SARS-CoV-2 variants
  • More infections among vaccinated, and possibly even boosted individuals, than seen with previous variants
  • Slightly higher rates of severe disease among non-boosted individuals older than 50 and immune-compromised people

If additional studies show that Omicron causes a decrease in vaccine effectiveness against severe disease and death, then Omicron-specific vaccines may become necessary.


Lisa Schnirring, Multiple early studies find notable Omicron vaccine evasion. December 8, 2021,

Jeremy Faust, DEFCON Omicron? Scientists report that Omicron evades antibodies. Here's what that might really mean. December 8, 2021,

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