While protection was still high, researchers found it declined by 13 percentage points in the period
A study funded by Pfizer-BioNTech showed the efficacy of its Covid-19 vaccine – while still high – declined by 13 percentage points over six months after the second dose, suggesting there could be a need for booster shots in the future.
More than 46,000 people in the United States, Turkey, Brazil, Argentina, South Africa and Germany were monitored for the study, which has not been peer-reviewed. Some 2,306 of the participants were aged between 12 and 15, while the rest were 16 or older. The median age was 51.
“Efficacy peaked at 96.2 per cent during the interval from seven days to two months post-dose two, and declined gradually to 83.7 per cent from four months post-dose two to the data cut-off – an average decline of 6 per cent every two months,” the authors – most of whom work for either Pfizer or BioNTech – wrote in the paper posted on preprint server medRxiv.org on July 28.
“Ongoing follow-up is needed to understand persistence of the vaccine effect over time, the need for booster dosing, and timing of such a dose,” they said.
Several vaccine makers are looking at whether extra doses for those who have been immunised are needed. It comes as countries around the world battle the highly infectious Delta strain, and those with high vaccination rates such as Israel have begun administering booster shots to protect against Delta and other new variants of the coronavirus.
A British public health study has found that protection from either of the two most commonly used Covid-19 vaccines against the now prevalent Delta variant of the coronavirus weakens within three months.
It also found that those who get infected after receiving two shots of either the Pfizer-BioNTech or the AstraZeneca vaccine may be of greater risk to others than under previous variants of the coronavirus.
Based on more than three million nose and throat swabs taken across Britain, the Oxford University study found that 90 days after a second shot of the Pfizer or AstraZeneca vaccine, their efficacy in preventing infections had slipped to 75 per cent and 61 per cent respectively.
That was down from 85 per cent and 68 per cent, respectively, seen two weeks after a second dose. The decline in efficacy was more pronounced among those aged 35 years and older than those below that age.
“Both of these vaccines, at two doses, are still doing really well against Delta … When you start very, very high, you’ve got a long way to go,” said Sarah Walker, an Oxford professor of medical statistics and chief investigator for the survey. Walker was not involved in work on AstraZeneca’s vaccine, which was initially developed by immunology experts at Oxford.
The researchers would not project how much more the protection would drop over time, but suggested the efficacy of the two vaccines studied would converge within 4-5 months after the second shot.
The Oxford findings are in line with an analysis by the US Centres for Disease Control and Prevention (CDC) and come as the US government outlines plans to make Covid-19 vaccine booster shots widely available next month amid a rise in Delta variant infections. It has cited data indicating diminishing protection from the vaccines over time.
US health officials are expected to advise Americans to get booster shots eight months after their second jab, Associated Press reported on Tuesday, citing sources. Formal approval of a third booster shot of the Pfizer-BioNTech vaccine is also expected in the coming weeks.
Meanwhile, countries like Turkey and Chile have begun offering booster shots of the Pfizer-BioNTech and AstraZeneca vaccines to people who have received Chinese Covid-19 jabs, amid concerns over their effectiveness against the Delta strain.
The Pfizer-BioNTech study did not recommend booster shots but noted that clinical trials were under way to produce more data on the safety and effectiveness of a third dose of the vaccine. It did not mention how protective the vaccine was against the Delta variant.
“Ongoing observation of participants through up to two years in this study, together with real-world effectiveness data will determine whether a booster is likely to be beneficial after a longer interval,” the paper said.
Benjamin Cowling, an epidemiologist at the University of Hong Kong, said the study provided compelling evidence that the Pfizer-BioNTech vaccine retains its efficacy for at least six months and likely for longer.
At the same time, he said booster shots could be useful. “I think there is evidence indicating that third doses will push immunity up to a higher level, and may be particularly beneficial for those who had a weaker response to initial vaccination,” he said.
The World Health Organization, however, has called for a moratorium on booster shots until at least the end of September so that vaccines can be supplied to poorer nations.
WHO director general Tedros Adhanom Ghebreyesus said earlier this month: “I understand the concern of all governments to protect their people from the Delta variant. But we cannot accept countries that have already used most of the global supply of vaccines using even more of it.”
While the WHO has no power to change the health policies of its member states, the message has been echoed by scientific journal Nature. “To focus on boosters when more than half the world lacks vaccine doses is short-sighted and will only keep the pandemic burning longer,” read an editorial in the journal published on Tuesday.
HKU epidemiologist Cowling agreed. “I think right now vaccine doses can do much more good if given as first, and second, doses in developing countries than if given as third doses in developed countries,” he said.
Additional reporting by Reuters