Double-jabbed who get Covid as likely to spread virus as unvaccinated - but less likely to catch it

Tom Clarke

Former Science Editor

ITV News Science Editor Tom Clarke reports on the latest major study about Covid vaccinations

A major study has found that while our Covid-19 vaccines continue to be brilliant at keeping people out of hospital and dying from the disease, we won't be able to rely on them to bring the pandemic under control.

The analysis looked into the effectiveness of the Pfizer and AstraZeneca vaccines against the Delta variant, compared to the previous Alpha (Kent) one.

We already knew neither vaccine was perfect at preventing someone getting infected with Covid. But the study conclusively shows that even if you're double-jabbed and you're unlucky enough to catch Covid, you might be just as infectious as you would have been if you'd never been vaccinated at all.

Vaccines are really good at preventing the vast majority of people from going to hospital. They also massively reduce the number of people who get infected and go on to infect others.

But what we now know is that they're not good enough to prevent the virus from spreading - especially right now when infection rates in the UK remain high. In short, the idea current vaccines will give us "herd immunity" from Covid is a myth.

"The fact that you can get [Covid], with high levels of virus in the nose and throat suggests that herd immunity may actually not be possible with this virus and the unvaccinated people may continue to still be at risk of hospitalisation and death, simply because there will always be virus floating around," said Professor Sarah Walker, who led the study at the University of Oxford. 

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You might ask, if vaccines are preventing most of us from getting sick and dying, why should that be a problem?

Well it's seriously bad news for anyone with a compromised immune system or otherwise vulnerable to severe Covid. They can't rely on others - like hospital and care home staff - being vaccinated to protect them. 

It also means that the virus is still able (though at a much reduced rate) to spread through the population and evolve.

The longer it does that in the presence of vaccinated people, the quicker we're likely to see another variant emerge that's even better at dodging vaccines.

And there was another, potentially concerning, finding from the latest analysis. It confirmed that the effectiveness of the vaccines at preventing infection decreases over time, particularly for the Pfizer vaccine.

Credit: PA

Two weeks after a second dose of Pfizer, protection against infection is 85%, but after three months it's declined to 75%. That's a 22% reduction in effectiveness per month. For the AstraZeneca vaccine it was a much more modest 7% decline.

It's nothing to be alarmed about yet. Firstly, because Pfizer is better than the AZ vaccine at preventing infection in the first place (85% v 68%), so the decrease is from a higher starting point. But if that level of decline continues at that rate, it suggests the Pfizer vaccine may have lower effectiveness in the long run.

"We see that AstraZeneca and Pfizer vaccines are behaving very differently in how that protection changes over time from second doses.

"There is no immediate cause for concern, both vaccines are doing really well, but they aren't the same," says Prof Walker.

The data strongly hints at the need for the government's booster vaccination programme sooner rather than later.