A third shot of the BioNTech/Pfizer Covid-19 vaccine offers protection against the Omicron coronavirus variant while just two doses show significantly reduced effectiveness against it, according to a preliminary study conducted by the vaccine makers.
Pfizer said in a statement on Tuesday that a third booster dose of the vaccine resulted in a 25-fold increase in the neutralising antibodies that attack the virus, compared with the original strain of the virus detected in China.
However, two doses may still induce protection against severe disease and the company is continuing to monitor the real-world effectiveness against Omicron.
Pfizer said it was developing a new vaccine targeted specifically at Omicron and expected to have it available by March, in the event that additional protection is required to combat the variant.
“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 maximised with a third dose of our vaccine,” said Albert Bourla, Pfizer’s chair and chief executive.
“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.”
Earlier preliminary studies conducted by academic institutions had shown Omicron was able to blunt the protective effects of antibodies from the vaccine but that inoculated people with additional protection from boosters or prior infection could retain some defences against catching the virus.
The studies, carried out by separate research teams in South Africa, Sweden and Germany, came to different conclusions about the extent to which Omicron undermines antibody protection. But they all suggested that people with just two doses of a Covid-19 vaccine were now much more susceptible to symptomatic infection.
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However, experts stressed that the findings did not mean the protection provided by the vaccines against severe illness would be weakened. The immune system draws on not just antibodies but also T and B-cells to defend against severe illness.
A study from Goethe University found that, for individuals who received two doses of the Pfizer vaccine at least six months ago, there was a near-complete loss of antibody protection.
The paper noted that “boosted individuals showed a significant increase” in antibody levels compared with double-vaccinated people, but protection was still markedly lower against Omicron than Delta. The loss of antibody protection for triple-jabbed people was comparable to that of double-vaccinated people who had also been previously infected.
Researchers from the Africa Health Research Institute reported that the loss of immune protection from the vaccine when it was pitted against Omicron was “extensive but incomplete” after analysing plasma samples from 12 participants.
The laboratory experiments found a 41-fold reduction in the ability of virus-blocking antibodies to neutralise Omicron, compared with the original strain of the virus detected in Wuhan almost two years ago
Importantly, the effect on vaccine efficacy is not known and will not be known for some weeks, as scientists still do not understand fully how antibody levels correlate with that measure.
Scientists at Sweden’s Karolinska Institutet said they had observed “exceptionally variable” loss of neutralisation against Omicron, with some samples “showing almost no loss” and some showing considerably more.
“There is clear agreement between the three studies that the Omicron variant is less susceptible to neutralisation by antibodies raised by either previous infection or a vaccine,” said Charles Bangham, professor of immunology at Imperial College London.
But he added that while this may lead to more breakthrough infections, T-cells would continue to provide a “good measure of protection against severe disease”.
The findings from all three studies, none of which has been peer-reviewed, raise the spectre of vaccines having to be reformulated to combat Omicron.
Additional reporting by John Burn-Murdoch in London and Peter Wells in New York