Does Covid immunity wane and will vaccine booster jabs be needed?
With plans for the UK’s Covid vaccine booster programme this autumn soon to be revealed, we take a look at what we do – and don’t – know about waning immunity after vaccination.
How is waning immunity against Covid investigated?
There are several ways to do this: one is to look at whether people who have been vaccinated become infected with Covid, and another is to look at levels of different components of the immune system in vaccinated individuals and see how these change over time.
Does immunity wane?
The answer from multiple studies seems to be yes, although it is worth remembering that no vaccine is 100% protective in the first place, and as more people are vaccinated, those who are fall ill are increasingly likely to have had the jabs.
But what is behind the apparent decline and what it means in practice, and for booster shots, is less clear.
Data from Israel, where 78% of people aged 12 and older are fully vaccinated, has shown a large increase in infection rates in recent months.
Meanwhile, data from the UK-based Zoe Covid Study app, where users can log whether they have been vaccinated, Covid test results and symptoms, has suggested protection against infection after two shots of the Pfizer/BioNTech jab decreased from 88% at one month to 74% at five to six months, while protection against infection after two Oxford/AstraZeneca jabs fell from 77% to 67% at four to five months.
Prof Tim Spector, the lead scientist on the study, said: “In my opinion, a reasonable worst-case scenario could see protection below 50% for the elderly and healthcare workers by winter.”
But as experts have pointed out, such findings are probably down to many factors, not simply that protection afforded by vaccination is wearing off.
Covid vaccines are somewhat less effective against the Delta variant than the Alpha variant. Delta is also more transmissible, which – together with policy changes such as relaxation of restrictions – means the chances of someone being exposed to the virus has risen. And there is the complication that those who were vaccinated first included the most vulnerable with weaker immune systems, who may be more prone to “breakthrough” infections.
Another factor is that the Zoe survey is based on a self-selecting sample of app users. “We have to be careful of confounders,” said Deborah Dunn-Walters, a professor of immunology at the University of Surrey, adding that the Zoe study team had also noted such issues.
Dunn-Walters said it was also possible many vaccinated people may not have symptoms, meaning their infection may go undetected.
But it is hospitalisations and deaths that are the biggest concern. “This latest [Zoe] study confirms that a decline is happening, but it is not yet clear what this means for disease severity, the key aspect of protection afforded by the vaccines,” said Prof Ian Jones, a professor of virology at the University of Reading.
Other studies have suggested biological components of immunity may wane over time, including work by UCL Virus Watch that suggested that in some cases antibodies generated in response to two doses of the Oxford/ AstraZeneca and Pfizer/BioNTech vaccines fell more than 50% over 10 weeks.
However, the World Health Organization (WHO) has said it is not clear if studies showing waning antibodies mean a decline in vaccine effectiveness.
What’s more, antibodies are only one component of the immune system, and experts say their decline is to be expected. Memory B cells and T-cells, which hang around for longer, are also crucial – but harder to measure.
“It is the memory B cells and the T cells that can be recruited back into any response very quickly to combat future infections,” Dunn-Walters said, adding that at least one study looking at immunity after vaccination with the Pfizer/BioNTech jab found memory B cells still present after six months.
Scientists are still working to understand what level of these different components of the immunity system is necessary to offer protection against infection and severe disease.
Does this mean booster shots are necessary?
For now, it seems the likely UK plan from the Joint Committee on Vaccination and Immunisation (JCVI) is to offer booster jabs only to particularly vulnerable people, such as those with weakened immune systems, meaning most people will not receive a third vaccination this autumn.
Prof Adam Finn, a member of the JCVI, told BBC Radio 4’s Today programme that studies suggested Covid vaccines still offered good protection against serious illness and hospitalisation.
“But we do need to watch out very carefully to see if this waning against milder disease begins to translate into occurrence of more severe cases, because then boosters will be needed,” he said.