Flu and other respiratory viruses seem to barely exist outside of winter, but covid-19 cases have consistently risen every summer over the past few years
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| Covid -19 is a risk all year around (Credit: Getty Images) |
Traditionally, summer is the season of sunburn and allergies, while winter is a time for sniffles and flu, but now it seems one pathogen is upending our yearly cycle of medical maladies: the coronavirus behind covid-19.
This infection also increases in summer, driving a recent rise in cases and hospitalisations in countries such as the UK, US and France. In England, for example, there were around 180 reported cases per week on average in early April, rising to more than 500 in mid-July.
Similar summer surges also played out in previous years. So, why does covid-19 seem to rise in summer, while other respiratory viruses – like flu and respiratory syncytial virus (RSV) – don’t?
Why does covid-19 have summer surges?
Covid-19 only emerged at the end of 2019, but a pattern has started to become clear: infections rise in both winter and summer. For example, the UK reported close to 7000 new cases a day at the end of 2022, which fell to around 300 in mid-July 2023 and increased to more than 1000 that August.
This is probably partly down to SARS-CoV-2, the virus that causes covid-19, mutating more rapidly than other common respiratory viruses, leading to new variants that our immune system may be less equipped to fight off. “It oftentimes takes a couple of years for significant immune-evading mutations to accumulate with flu, and with RSV it happens very rarely, but with covid, it seems to be happening multiple times within a particular year,” says Andy Pekosz at the Johns Hopkins Bloomberg School of Public Health in Maryland.
Despite new variants sometimes compromising our immunity, our ability to fight them off is generally not completely depleted if we have been vaccinated or infected with another variant before. Nevertheless, SARS-CoV-2 is a relatively new infection, so people haven’t developed long-term immunity to it, says Marc-Alain Widdowson at the World Health Organization. “With the vast majority of respiratory infections, your immune system is kind of branded with them as a kid through repeated infections over several years,” he says.
This weaker immunity paired with SARS-CoV-2’s rapidly mutating nature also means cases are at a higher baseline level throughout the year than flu or RSV. As a result, “you would expect surges to start and move at a faster level”, says Pekosz.
It isn’t entirely clear why certain infections are most prevalent in winter, but it could partly be due to people generally socialising indoors during the cold months, aiding transmission. Although warmer, summer is also often a sociable time of year, when people may travel, says Francois Balloux at the University College London. Such conditions could increase the transmission of any respiratory infection, but having SARS-CoV-2 cases exist at a higher baseline may prompt more infections in summer than other viruses, says Balloux.
Are summer surges here to stay?
The experts are unanimous that covid-19 hasn’t existed for long enough to reliably say if or when case surges will settle into a reliable pattern. But Widdowson suspects that, in time, they will probably cluster in winter, because that is the pattern with other coronaviruses that cause common cold-like symptoms.
But Balloux says the rapid emergence of new SARS-CoV-2 variants makes it particularly difficult to gauge when covid-19 may ebb and flow long term. “I think we’ll see,” he says.
Like other respiratory viruses, SARS-CoV-2 clearly spreads most in winter, so the largest annual surge will almost definitely continue to occur then, says Pekosz. “I think we just need to really keep monitoring the virus to see if it follows our wishes and fits into a pattern that makes it more easy for us to deal with it.”
Should this influence vaccinations?
Earlier this year, several countries launched spring covid-19 vaccination campaigns. However, despite around 7 million people being eligible for a vaccine on England’s National Health Service, only just over 4 million were administered, for example.
“Summer surges, particularly causing severe disease in vulnerable populations, may mean that we have to do more to mitigate disease in the summer to really help target those populations,” says Pekosz.
With SARS-CoV-2 having a relatively high baseline of cases throughout the year, getting vaccinated in any season would probably have benefits, says Widdowson.
But the most recent spring vaccines had the same formulations as those developed for previous autumn roll-outs, making them less useful against new variants. Vaccines can take months to develop, so manufacturers may be unable to make several versions every year, says Pekosz. But if covid-19’s summer surge is here to stay, such efforts may be necessary, he says.
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