Getting to grips with . . .

What the new Oxford-ONS study reveals about the effectiveness of Covid vaccines

Largest study of its kind suggests vaccinated people carry same levels of virus as the unvaccinated

The effectiveness of the Pfizer--BioNTech vaccine declines faster than that of the Oxford-AstraZeneca jab - but neither reduces the amount of Covid-19 carried by a person once infected, according to a major new study.

Researchers at Oxford University and the Office for National Statistics (ONS) analysed 2.5m test results from 743,526 UK participants in a coronavirus household-infection survey. 

The publication of the study findings coincides with a meeting today of the Joint Committee on Vaccination and Immunisation (JCVI) to discuss a potential Covid jabs booster campaign. The study’s authors - who were not involved in the making of the AstraZeneca shot - said that based on the results of their analysis, the government may need to “think about having different policies moving forward for different vaccines”. 

Viral loads

Infected people who are doubly vaccinated carry the same amount of Covid - known as viral load - as the unvaccinated, according to the new study.

Although previous research showed that the vaccines reduced viral load for the Alpha variant of coronavirus, this reduction appears to have been wiped out by the Delta variant, now the dominant strain in the UK.

However, the jabs still offer protection against symptomatic infection, and fully vaccinated people are less likely to contract Covid in the first place.

The viral load finding “calls into question the effectiveness of vaccine passports and changes to the NHS app” that mean fully vaccinated people no longer need to self-isolate after contact with a positive case, said The Telegraph.

“The new results suggest there could still be a risk even among the fully vaccinated,” the newspaper continued. But the researchers have emphasised that the transmission risk in fully jabbed people is not yet clear.

Waning effectiveness for Pfizer

The study also found that the Pfizer vaccine was more effective than the AstraZeneca version against symptomatic infection from the Delta variant for four to five months after receiving the second dose. But the efficacy of the Pfizer jab then almost halved, putting it roughly on par with the AstraZeneca vaccine, which had barely decreased.

The Financial Times said the study findings are in line with research in Israel, which also indicates declining Pfizer efficacy. Tomas Hanke, professor of vaccine immunology at Oxford’s Jenner Institute, told the newspaper that the AstraZeneca jab might offer longer-lasting immunity because the spike protein generated by the vaccine stays in the body for more time, while the finite number of mRNA molecules delivered by the Pfizer vaccine “are eventually cleared from the system”.

The new research appears to support the argument for a booster Pfizer shot, but some experts have warned that there might be other variables at play. For example, the people vaccinated early in the pandemic tended to be more vulnerable so may have less robust immune responses.

Professor Sarah Walker, chief investigator and academic lead for the Covid-19 Infection Survey, said policymakers may need to take into account “which vaccine you had initially and how long the protection from that lasts” when deciding who should receive a third shot.

Time between doses irrelevant 

Another key finding of the study is that the time between doses does not impact the effectiveness of preventing new infections. 

Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading, told the i news site that “these datasets scotch claims that protection provided by the UK’s vaccination programme is likely to be superior to Israel’s because of our longer window between first and second doses”.

The new research also found that younger people (aged 18-34) gained more protection from vaccination than older age groups (35 to 64-year-olds).

Dr Koen Pouwels, senior researcher at the University of Oxford’s Nuffield Department of Population Health, told the i site that “even with these slight declines in protection against all infections and infections with high viral burden, it’s important to note that overall effectiveness is still very high because we were starting at such a high level of protection”.

“It is also worth highlighting that these data here do not tell us about protection levels against severe disease and hospitalisation, which are two very important factors when looking at how well the vaccines are working,” he added.

And given those factors, the BBC’s senior health reporter Laura Foster concluded, “having two doses of Covid vaccine remains the best way to protect against the Delta variant”.


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