With recent UK data suggesting that the BA.4 and BA.5 Covid variants are kicking off a new wave of infections, experts answer the key questions about reinfection and prevention.
Though rare at the start of the pandemic, reinfections have become increasingly common as the months and years wear on – particularly since the arrival of Omicron, which prompted a 15-fold increase in the rate of reinfections, data from the Office for National Statistics suggests.
In part, this is because of a decline in protective antibodies triggered by infection and/or vaccination over time, but the virus has also evolved to evade some of these immune defences, making reinfection more likely.
“The original Omicron BA.1 variant was itself massively immune-evasive, causing a huge breakthrough caseload, even in the vaccinated,” said Danny Altmann, a professor of immunology at Imperial College London. “It is also poorly immunogenic, which means that catching it offers little extra protection against catching it again. On top of that, there’s now further evidence of the very marginal ability of prior Omicron to prime any immune memory for BA.4 or 5, the sub-variants that seem to be driving the latest wave of infections.”
The virus has also evolved to become more transmissible, meaning even fleeting exposure to an infected person means you may inhale enough viral particles to become infected yourself.
“There are definitely a lot of people who got Covid at the start of the year who are getting it again, including some with BA.4/5 who had BA.1/2 just four months ago, who thought they would be protected,” said Prof Tim Spector, who leads the Zoe Health Study (formerly known as the Zoe Covid Study).
“We still don’t have enough data to work out exactly when the susceptible periods [for reinfection] are, which is one reason why we need people to keep logging their symptoms. We do know it’s still quite rare within three months, and it used to also be rare within six months, but that’s not the case any more.”
According to unpublished data from Denmark, which looked at reinfections with the BA.2 Omicron sub-variant within 60 days of catching BA.1, such reinfections were most common among young, unvaccinated people with mild disease. Other studies have similarly suggested that Covid-19 vaccination provides a substantial added layer of protection against reinfection by boosting people’s immune responses.
However, Omicron infection in itself appears to be a poor booster of immunity, meaning that if you were infected during earlier pandemic waves, your immune response is unlikely to have been strengthened by catching it again earlier this year.
In general, infections should be less severe the second, third or fourth time around, because people should have some residual immunity – particularly if they’ve also been vaccinated, which would further raise their levels of immune protection. However, there are always exceptions to this. “Anecdotally, some people are getting it for longer this time around than they did the last time,” Spector said.
It is also too early to know about the risk of long Covid associated with BA.4/5, he added.
As the UK heads into a period dominated by BA.4 and 5, the potential for reinfection seems high. “We’re in quite a serious situation due to a convergence of factors: a country where a moderately successful third booster campaign is now long past, with immunity waned and successive large waves of Omicron through to the emerging dominance of BA.4/5,” said Altmann.
“The bottom line is that we should all consider ourselves essentially unprotected, except perhaps from intensive care unit admission and death, and then, as before, with the risks increasing with age.”
Face masks and ventilation continue to provide important additional layers of protection – especially in crowded settings. “I still wear a mask, but not a cheap mask – I wear a proper FFP2 or 3 mask,” said Spector. “These new variants are still very much airborne and you need an even smaller amount to get infected, so I think a mask is definitely a good idea when as many as one in 30 people have it again.”