A new British study supports data that shows the BioNTech/Pfizer coronavirus vaccine becomes less effective over time — a finding that could support governments’ decisions to offer booster shots to their populations.
The BioNTech/Pfizer shot initially offers better protection than the Oxford/AstraZeneca jab, preventing up to 85 percent of infections two weeks after the second shot. But the mRNA shot’s efficacy decreases more rapidly, dropping to 75 percent after 90 days, according to a preprint of the study conducted by Oxford University researchers, which has yet to be peer-reviewed.
The Oxford/AstraZeneca shot, on the other hand, is 68 percent effective at preventing all COVID-19 cases two weeks after the second shot, which decreases to 61 percent after three months.
Based on Oxford’s modeling, “after around four to five months, the two vaccines would be similar” in how much protection they offer, said Sarah Walker, a professor of medical statistics and epidemiology at the University of Oxford, who is also the chief investigator for the U.K.’s National COVID-19 Infection Survey.
The Oxford study looked at infections after taking 2.5 million test swabs from the start of December until August. Its authors stressed to reporters on Wednesday that both vaccines are highly effective against the Delta variant — even if the efficacy of the BioNTech/Pfizer jab decreases faster.
“Remember,” said Walker, “protection starts from a very high level.”
Debate over booster shots is increasing after the World Health Organization called for a moratorium on giving extra jobs until at least 10 percent of every country is vaccinated by the end of September — a goal that the world is far from reaching.
Wealthy countries are still going ahead. Israel and Hungary began giving third doses this summer, and the U.S. announced plans to follow suit this fall. Other countries, mostly in the EU, are only offering third shots to people who are immunocompromised or at risk, a move largely in line with the WHO’s recommendations.
Penny Ward, professor of pharmaceutical medicine at King’s College London, wrote in a statement that Oxford’s study could lend support to calls for booster shots for people who received two mRNA shots. Similarly, she suggested people who received two doses of the Oxford/AstraZeneca vaccine could get an mRNA shot to “enhance protection against this variant as winter approaches.”
Researchers cautioned against drawing any conclusions from the study, especially about the differences in the two vaccines as that must be assessed over time. They also pointed out that the study looked at all infections — including asymptomatic ones — and did not differentiate between severe COVID-19 cases, nor did it assess hospitalizations or deaths.
“The main value of immunization is in reducing the risk of severe disease and death and the evidence available shows that protection lasts longer against severe disease than against mild disease and all current U.K. vaccines are very good at this, even against the Delta variant,” Paul Hunter, a professor of medicine at the University of East Anglia, wrote in a statement.
Another key finding from the study was that the Delta variant might impact vaccinated people’s ability to transmit the virus.
People who contracted coronavirus after being vaccinated with two shots showed similar viral loads to unvaccinated people who had contracted the virus. Previously, when the Alpha variant was dominant, vaccinated people showed a much smaller viral load.
The study’s researchers were hesitant to make any claims about transmission, however. In one sense, vaccines are able to shrink transmission rates because they greatly decrease the chances of someone getting infected. But if someone does become infected while vaccinated, their high viral load means there’s still a risk that person could pass the virus on to others, according to Koen Pouwels, a senior researcher at Oxford’s public health department who took part in the study.
Pouwels added that the study offers a “hint” that the vaccines “are probably best at preventing severe disease and slightly less at preventing transmission.”
“We don’t yet know how much transmission can happen from people who get COVID-19 after being vaccinated,” Walker said. “But the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped.
“This means it is essential for as many people as possible to get vaccinated — both in the U.K. and worldwide.”
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