Researchers say they are seeing a decrease in protection against Covid-19 infections in people who have received both doses of the vaccine.
One study, conducted in the UK with real-world data, looked at positive PCR test results between May and July 2021 in more than one million people who had received two doses of the Pfizer or AstraZeneca vaccine.
Protection after two doses of Pfizer decreased from 88% in one month to 74% in five to six months. In the case of AstraZeneca, the drop went from 77% to 67% in four to five months.
THE reduced protection is expected, according to the experts.
Although there are cases of infection in fully immunized individuals, vaccines are still doing a good job of protecting the population against the severe form of the disease and deaths from Covid-19.
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Public Health England, the British government’s public health agency, estimates that around 84,600 deaths and 23 million infections were prevented as a result of the Covid-19 vaccination campaign in England so far.
Professor Tim Spector of King’s College London (KCL), who led the study based on data from the epidemiological research app Zoe Covid, says the findings could explain the recent infections that some fully vaccinated people have reported.
“The decrease in protection is expected and is no reason not to be vaccinated,” he says.
“Vaccines still provide high levels of protection for the majority of the population, especially against the Delta variant, so we still need as many people as possible to be fully immunized.”
He estimates that protection against infections could drop to 50% in winter and booster shots will be needed — but other experts urge caution when making predictions for the coming months.
The expectation is that the UK will start offering some people a third booster dose of covid vaccine next month, but the government is awaiting the recommendations of an independent advisory body, called the JCVI, which is analyzing evidence to support the decision.
“Many people may not need it. Many people may have received a natural booster because they already had a natural covid infection, so, effectively, they will have taken three vaccines,” says Spector.
“So I think the whole thing needs to be managed with much more care than just giving (the third dose) to everyone, which would be a huge waste and ethically dubious given the resources we have. I think we need a more focused approach than last time.”
Simon Clarke, an expert in cell microbiology at the University of Reading, UK, says that levels of infection in the community would alter the chance that a person will come across (with the virus) and catch covid at any time, making it difficult to draw definitive conclusions about the decline of immunity.
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Alexander Edwards, also of the University of Reading, says it’s important to understand when booster shots might be needed and for whom.
“Vaccination does not make people invulnerable – and it does not prevent all infections. The strains have a real and significant impact on public health, and many people are still tragically dying in the UK from this horrible virus.”
“The vaccines we have are remarkably safe and effective, and they’re still far better than other vaccines that offer enormous benefits.”
“We must proactively plan our public health strategy to take into account imperfect protection and the possibility of declining protection over time,” he adds.
A similar study was published by the Office for National Statistics and the Oxford Vaccine Group last week.
Based on the results of the PCR test of nearly 400,000 people infected with the Delta variant in the UK, it showed that two doses of Pfizer vaccine were initially more than 90% effective against symptomatic covid infection, compared to around 70% in the case of the AstraZeneca vaccine.
But over the course of three months, Pfizer’s protection dropped significantly, while AstraZeneca’s immunity remained more stable.
Professor Adam Finn, a government adviser on vaccines, said other studies had shown that vaccines maintain good protection against serious illnesses and hospitalization.
But he added: “We need to be very careful to see if this reduction (in immunization) for milder diseases starts to translate into more severe cases, because then booster shots will be needed.”
For now, there is still no definition on the booster vaccine in Brazil, but both the Ministry of Health and the Butantan Institute (responsible for finalizing the production of CoronaVac in the country) admit that they are evaluating and considering this possibility.