Covid vaccines and immunity: does past infection help?

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T he first phase of the NHS vaccination programme aims to protect millions of the UK’s most vulnerable people against coronavirus. Meanwhile, research from Public Health England suggests that a previous Covid infection might protect as much as a vaccine, at least for five months. So what difference does it make to have such immunity?

Can I still catch the virus if I have had the vaccine or been previously infected?

You can still catch the virus if you’ve had the vaccine or a previous coronavirus infection. Clinical trials for the Covid vaccines approved so far show that they can substantially reduce the risk of becoming ill with the virus, but some people may still get infected and even be able to spread the disease. The same is true for a previous infection.

A Public Health England study of healthcare workers found that Covid infection led to about 83% protection against reinfection for at least five months. That still means almost two out of every 10 people could become reinfected within five months of their first skirmish with the disease. Some who became reinfected had high enough viral loads to pass the disease on. More encouragingly, less than a third of those who were reinfected had symptoms the second time around, compared with 78% the first time.

Deborah Dunn-Walters, professor of immunology at the University of Surrey and chair of the taskforce on immunology and Covid-19, run by the British Society for Immunology, said: “Your immune system could work at 100% and protect you from disease and the virus, or it could work less well and protect you from the disease, but not stop you getting and transmitting the virus on. Or it could work even less well, where you still get ill, but not as bad as you would have done.”

Should I still have the vaccine if I’ve had Covid?

Yes. The immune system’s response to coronavirus infection varies from person to person. Age, genetic makeup and the amount of virus you were exposed to all play a role. The variability makes it hard to predict how well a particular person will be protected after infection. A benefit of the vaccines is that they have gone through trials to assess how well standard doses prevent disease, so there is far more certainty over the protection that they confer and the impact they will have on public health.

Another point is that many people who believe they caught Covid last year have no proof of it. Is it safe to be vaccinated after having Covid? “There is no harm in giving the vaccine to a person who has had the disease – it will act as a booster,” said Dunn-Walters.

Should people who have had Covid be last to get the vaccine?

No. This would complicate an already highly complex vaccination programme. Healthcare workers are at high risk of infection and many have already caught the disease. The vaccines will boost their natural immunity, making them less likely to become reinfected. Others at the front of the queue are older people. Their immune systems tend to be weaker, so even if they have recovered from Covid-19 they remain vulnerable and the vaccines will bolster their resistance to reinfection.

The uncertainties about who is well protected or not after an infection makes it hard to justify sending any group to the back of the queue, regardless of age and health. “For the time being, everyone should be offered the vaccine, whether or not they have previously had the disease,” said Eleanor Riley, professor of immunology at Edinburgh University.

Can I see my grandchildren after having the vaccine?

As vaccines are administered, public health authorities will get a clearer picture of how well they protect older people. Riley recommends waiting for about a month after the second dose to ensure that the immune response is as strong and long-lasting as possible.

There is still some uncertainty about how well the vaccines will protect against some of the new variants of coronavirus, but scientists hope to have a better idea by the time the lockdown is lifted. “We very much hope, and expect, that it will be safe for people to see, and to hug and kiss, their grandchildren in the very near future. But we need to proceed cautiously,” Riley said. Dunn-Walters warned that grandparents could potentially still spread the virus after being vaccinated.

When do we get back to a more normal life?

Riley says she believes the route back to normality is to ensure, first, that everyone at risk of developing severe Covid-19 is vaccinated, and then that those at risk of developing symptomatic disease and long Covid are vaccinated.

“It is very likely that we can achieve this for the UK population by the summer for the first group – those over 50 years of age, and those under 50 with underlying health conditions – and by the end of the year for the second group, all other adults over the age of 18.”

The first sign that the strategy is working will be a fall in hospital admissions and deaths, and if the vaccines prevent transmission as well as illness, then a fall in new infections as the vaccines are given to less vulnerable adults.

“This should allow life in the UK to return to something close to normal. However, if new virus variants emerge that are able to escape the vaccine, this could delay things. This will be evident if we start to see people becoming sick with variant viruses despite having been vaccinated,” Riley said.