To arrive at that conclusion, they analyzed 620 blood samples from 364 health care workers and staff at the Francis Crick Institute and the National Institute for Health Research UCLH Biomedical Research Centre, in London.

People who had received only two doses of either the AstraZeneca vaccine or the Pfizer vaccine were less protected against the Omicron variant than against the Alpha and Delta variants. Antibody levels declined in the first three months after the second dose, but a third (booster) dose increased levels of antibodies that combat the Omicron variant.

In people who received the Pfizer vaccine for all three doses, antibody levels against Omicron after a third dose were similar to levels against Delta after two doses. Overall, antibody levels were nearly 2.5 times higher against Omicron after three doses than after two doses, the investigators found.

Higher levels of antibodies against the Omicron variant were also found in people who received two doses of either the AstraZeneca vaccine or the Pfizer vaccine and previously had COVID-19 symptoms, compared to those without previous COVID-19 symptoms.

Antibody levels alone do not predict vaccine effectiveness, but they are a good indicator of protection against severe COVID-19 and hospitalization, according to the team that conducted the research as part of what's known as the SARS-CoV-2 Legacy study.

"People who have queued outside vaccinations centers should be reassured that a vaccine booster is the best way of protecting them from Omicron. And for people who haven't yet had a booster or even a first dose, it's not too late," said Emma Wall, a UCLH infectious diseases consultant and senior clinical research fellow for the Legacy study.

"This new variant can overcome the immune blockade put in place by two vaccine doses, but thankfully following the third dose, neutralizing activity is robust in the vast majority of people. A third dose builds our defenses higher, making it harder for the virus to cause severe COVID-19," Wall explained in a Crick Institute news release.

The findings were published online Jan. 19 as a research letter in The Lancet medical journal.

"While the Omicron variant has considerably more mutations than other recent variants, such as Alpha and Delta, our data show that the boosters push our immune system to make a broad response capable of tackling it," said David Bauer, group leader of the Crick's RNA Virus Replication Laboratory and member of the U.K.'s Genotype-to-Phenotype National Virology Consortium.

Sonia Gandhi is Legacy chief investigator at Crick and a consultant neurologist at UCLH. She said, "Now that we have established that boosters are effective against the Omicron variant, future research will need to address the duration and persistence of this booster response. New variants of concern will continue to emerge as the pandemic evolves, so effective immune monitoring is needed to stay responsive and remain protected."