Long-term efficacy of vaccines in preventing Covid-19 is questioned

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The rise in the number of people who have contracted the coronavirus has raised doubts about the lasting effectiveness of Covid-19 vaccines, according to new studies, including one that found that protection gained from a BioNTech/Pfizer dose waned more quickly than that from AstraZeneca jab.

An Oxford University study published Thursday found that the effectiveness of the Pfizer vaccine against symptomatic infection was nearly halved after four months, and that people vaccinated with the most contagious Delta virus had as high viral loads as the unvaccinated.

Two papers from the US and Qatar also raised the debate about the need for additional booster doses as they found greater numbers of “superbugs” than expected, although protection against serious cases of the virus appears to be holding up.

The spread of the delta variant has made stopping transmission “more difficult,” said Natalie Dean, a professor of biostatistics at Emory University.

“The situation has changed in terms of how far we think vaccines can take us,” she said. “We are brought back to a more modest – but still crucial – goal: preventing severe disease, hospitalization and mortality.”

What do the latest studies show?

Oxford scientists have shown a drop in vaccine efficacy since the Delta strain became dominant in the UK in May. While the Pfizer syringe was most effective at first, within four to five months after the second dose, its efficacy was roughly the same as that of AstraZeneca, with the protection afforded by the latter barely budging.

The paper’s authors were not involved in creating the AstraZeneca vaccine, which originated at Oxford University.

Thomas Hanke, professor of vaccine immunology at the Jenner Institute in Oxford, speculated that an injection of AstraZeneca generates long-lasting immunity because its high protein stays there longer, promoting a greater immune response.

“When you introduce RNA, like the Pfizer vaccine, you introduce a limited number of mRNA molecules that are eventually eliminated from the system,” he said. “But when you deliver the adenovirus, as AstraZeneca does, you introduce a model that then goes on to produce mRNAs that then produce the spike protein, so there’s no ceiling.”

An initial version based on evidence collected at the Mayo Clinic chain of hospitals in Minnesota showed that protection against infection fell from 91 percent to 76 percent between February and July for a vaccine made by Moderna, and from 89 percent to 42 percent for a cent cake. Pfizer.

It is unclear how much of this was a result of the delta variant, which was absent in Minnesota in February but prevailed by July, and how much was due to diminished immunity as the months passed after people were vaccinated.

A separate country study focused on the delta variant found that two doses of Pfizer were 60 percent effective in stopping infections, whether they were symptomatic or not, while Moderna was 86 percent effective.

People wait in line to receive their BioNTech/Pfizer or AstraZeneca vaccines in Valparaíso, Chile. © Rodrigo Garrido / Reuters How does this compare to what we already knew?

Real-world studies by Public Health England in May painted a more rosy picture: Pfizer’s dual vaccine was 88 per cent effective in preventing symptomatic delta-variable infections. Studies in Canada and Scotland put the efficacy at 87 percent and 79 percent, respectively.

But the new studies seem more in line with research in Israel, which found that the Pfizer injection was only 41 percent effective in preventing symptoms in June and July. The Oxford study was the first to suggest that the effectiveness of a Pfizer dose may wane faster than AstraZeneca.

However, direct comparisons between different studies are difficult. The US and Qatar papers included people who did not show symptoms – and they are known to produce lower estimates of effectiveness.

However, Laith Abu-Raddad, author of the country paper and professor in Weill Cornell Medicine, part of Cornell University, said the results were “absolutely surprising” because scientists were more concerned about efficacy against the beta variant rather than the delta strain.

Is this the waning of immunity over time?

The reason for the reduced efficacy could be weak immunity, which supports the argument for the third shot. Studies have shown that antibody levels decrease over time, although scientists have not determined at what level the protection stops. Other parts of the immune system, such as the hard-to-watch T cells, also play a role in fighting the virus.

Abu Raddad said his study indicated a waning of immunity but cautioned that “the sky is not falling,” given that vaccines still prevent severe disease.

They took into account a “long list” of complicating factors, so it was reasonable to assume that the drop was due to weakened immunity, said Quinn Boyles, lead author of the Oxford study.

Pfizer has said for some time that a third shot will be necessary, perhaps about eight to 10 months after the second dose. I have applied to several regulators for approval for a booster dose.

Adam Fane, a member of the British Joint Committee on Vaccination and Immunization, said there was “no clear evidence” of the need for a booster dose and urged caution, especially when some companies have “a strong financial incentive to propose a boost”.

Health care workers prepare doses at a mass vaccination site inside Darlington Racecourse in South Carolina © Micah Green / Bloomberg Does Moderna’s vaccine offer more protection against Delta?

Studies have also shown that Moderna’s vaccine may be more effective in treating the delta variant than Pfizer’s vaccine.

In the Minnesota study, Pfizer’s efficacy against infection declined more sharply as the delta variant rapidly replaced alpha as the dominant strain. One complication is that the Pfizer vaccines were given first and Moderna was recently rolled out, but the researchers tried to make up for this by comparing groups that were only vaccinated in the same month.

The Oxford researchers had only enough data to study the effect of the first dose of Moderna’s vaccines, but they found it to be as effective as or greater than a single dose of the other vaccines.

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The Qatar study showed a much lower efficacy rate for the Pfizer dose but did not adjust for how long each individual was vaccinated. However, Abu Raddad said he did not believe this could be the only explanation.

“They are both really great vaccines, but there may be a difference in the dose,” he said. Moderna’s vaccine contains three times more mRNA — the genetic instructions that teach the body to recognize the spike protein — than the Pfizer vaccine.

Will these studies change the rules of the game?

Experts have been reluctant to draw exciting conclusions from these new studies because there are so many other variables.

People who were vaccinated early tend to be more at risk, so they may have been less likely to have strong immune responses anyway.

“How old are you, the underlying risk factors that predict you’ve been vaccinated long ago can also be the reason why vaccines fail and you get sick,” Professor Finn said.

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Muge Cevik, an infectious disease researcher at the University of St Andrews in Scotland, said it will be increasingly difficult to interpret such studies due to complex factors including changing behaviour, such as after lockdown is lifted, or developing immunity following an infection.

Even without these shifts muddiing the waters, it’s possible that vaccines are less effective simply because, over time, vaccinated people come into contact with the virus over and over again.

“The higher the rate of transmission you have, the more likely you are to get a hack case,” said computational geneticist Yaniv Ehrlich. “Someone rolls the dice once, twice, three times and maybe the third time they roll the wrong number and get [infected]. “

Additional reporting by Oliver Barnes in London

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