They also found that antibody levels dropped off in the first three months following the second dose but a third ‘booster’ dose raised levels of antibodies that effectively neutralise the Omicron variant.
The study found that in people who had received the Pfizer vaccine for all three doses, antibody levels against Omicron after a third dose were similar to those previously reached against Delta after only two shots.
Overall, antibody levels were nearly 2.5 times higher against Omicron after three doses compared to after two jabs, according to the researchers.
Higher levels of antibodies against Omicron were also found in people who received two doses of either vaccine and also reported previously having COVID-19 symptoms, compared to those who didn’t have disease symptoms in past.
While levels of antibodies alone do not predict vaccine effectiveness, they are a very good indicator of protection against severe COVID-19, the researchers noted.
The study confirms that three doses of COVID-19 vaccine are essential to boost antibodies to quantifiable levels and maximise the amount of protection against severe disease and hospitalisation.
“People who have queued outside vaccinations centres should be reassured that a vaccine booster is the best way of protecting them from Omicron,” said Emma Wall, an infectious diseases consultant at UCLH Biomedical Research Centre of NIHR.
“And for people who haven’t yet had a booster or even a first dose, it’s not too late,” Emma said in a statement.
The researchers analysed 620 blood samples from 364 people who enrolled in the study.
They tested the ability of antibodies to block entry of the virus into cells, so called ‘neutralising antibodies’, against different variants of SARS-CoV-2, including Omicron.
The researchers also included synthetic neutralising antibodies that are currently in use for COVID-19 treatment to test if they have neutralising activity against variants of SARS-CoV-2.
The team found that Xevudy (sotrovimab), a recently-approved synthetic monoclonal antibody used to prevent and treat patients at risk of developing severe COVID-19, was able to neutralise the Omicron variant.
“We have really important early data to suggest that at least some versions of synthetic antibodies that we currently use to treat certain patients, are likely to be effective against this new variant,” Bryan Williams, UCLH Director of Research, added.
David LV Bauer, from Francis Crick Institute, said while Omicron has considerably more mutations than other recent variants, such as Alpha and Delta, the study shows that the boosters push the immune system to make a broad response capable of tackling it.
“This new variant can overcome the immune blockade put in place by two vaccine doses, but thankfully following the third dose, neutralising activity is robust in the vast majority of people,” Wall said.
“A third dose builds our defences higher, making it harder for the virus to cause severe COVID-19,” she explained.