The data, which has not yet been peer reviewed or published, shows that Omicron can evade immune responses better than previous variants.
Researchers collected blood samples from people vaccinated with two doses of Pfizer’s vaccine as well as those who received a booster shot. They analyzed how antibodies and other key immune cells fared against Omicron in a laboratory setting.
In blood samples of those who only received the primary series, neutralizing antibody levels fell 25-fold against the new variant. But in people who received three doses of the Pfizer vaccine, the antibodies generated were similar to those from two doses against the older strains of the virus.
“The bad news is that there appears to be diminished protection with two doses compared with previous variants,” Leana Wen, MD, a public health professor at George Washington University, told Verywell. “The good news is that the third dose—the addition of a booster—gives an added level of protection that essentially restores the protective effect of the vaccine.”
“This is in line with what the predictions have been all along and also with the recommendation for everyone to get their boosters at this point,” she added.
Pfizer’s preliminary data seems to support findings from another early lab test conducted by scientists in South Africa, which showed a 41-fold decrease in antibody levels against Omicron for people with two doses of Pfizer vaccine. This means individuals who have recovered from COVID-19 could be reinfected, and vaccinated people could be more prone to breakthrough infections.
Omicron appeared to cause a “much more extensive escape” from virus-fighting antibodies than has been observed by other variants in similar studies, according to the scientists. Still, the authors said that the variant did not totally thwart the vaccine elicited immunity.
Plus, antibody levels in samples of people who had recovered from COVID-19 were high enough to protect against Omicron, compared to those who had no prior infection.
Though neutralizing antibodies are key to fighting viruses like COVID-19, they are only one player in the larger immune system. T-cells, which kill infected cells, may still attack the variant. Omicron contains some mutations in the spike protein—the part of the virus that grants it entry into human cells. But for the most part, T-cells induced by the vaccine will still be able to recognize the virus’s binding sites, Pfizer said. This may mean the vaccines can prevent severe illness, despite the drop in antibody levels.
“Although two doses of the vaccine may still offer protection against severe disease caused by the Omicron strain, it’s clear from these preliminary data that protection is improved with a third dose of our vaccine,” Albert Bourla, chairman and chief executive officer of Pfizer, said in the statement. “Ensuring as many people as possible are fully vaccinated with the first two dose series and a booster remains the best course of action to prevent the spread of COVID-19.”
Researchers are still studying how the available vaccines hold up against the Omicron variant in clinical and real-world studies. In the meantime, vaccine manufacturers including Pfizer, Moderna, and Johnson & Johnson are developing Omicron-specific vaccines.
The new data, Wen said, reinforces the message that people should receive a booster shot as soon as possible. Individuals 18 years and older can get an additional dose of Pfizer’s or Moderna’s vaccine six months after their primary series.
The additional dose also increases protection against the Delta variant, which is responsible for more than 99% of COVID-19 cases in the United States.
“The most important thing that people should guard themselves against is still the Delta variant,” Wen said. “The same things that protect against Delta will protect against Omicron. Specifically, getting a booster dose is urgent and essential. It’s not just a nice to have—it’s really crucial.”
