Nearly five years after Covid-19 reached the UK, the virus is in its fourth wave of 2024, with hospitalisations surging by more than 20 per cent in a week.
Hospital admissions are much lower now than they were earlier in the pandemic while an infection is generally far less serious. But new waves of the virus keep coming back and the results can still be severe, posing the risk of serious illness, death and long Covid, especially among the clinically vulnerable.
By contrast, respiratory syncytial virus (RSV) – a key cause of the common cold – and flu are seasonal viruses, meaning you can relax in the spring and summer and start worrying about them in the autumn.
Covid, meanwhile, is year-round, with the UK suffering two waves this summer alone, in June and July.
So why does Covid keep coming back and how much longer is this going to go on for?
The conveyor belt of waves is driven by a constant stream of new variants and sub variants – working in conjunction with waning immunity against the virus – and the fact that Covid is intrinsically about three times more contagious than flu and RSV.
So a new variant comes along that is able to evade existing immunity built up from previous infections and vaccines better than are existing variants, which pushes up cases and can fuel a new wave.
In the following weeks and months, the wave of infections pushes up the nation’s immunity against the new variant and cases subside.
But that immunity starts to decline quite quickly and, because Covid is fundamentally much more contagious than flu and RSV, it doesn’t take much to start up a new wave.
“The problem with Covid is that it evolves so quickly. Its evolutionary rate is something like three or four times faster than that of the fastest seasonal flu,” professor Steve Griffin, of Leeds University, told i.
“So you’ve got this constant change in the virus, which accelerates the number of susceptible people. It’s creating its own new pool of susceptibles every time it changes to something that’s ‘immune evasive’.
“Every one of these subvariants is distinct enough that a whole swathe of people are no longer immune to it and it can infect them. That’s why you see this constant undulatory pattern which doesn’t look seasonal at all,” he says.
Adding in seasonal factors, such as cooler weather forcing people indoors and children returning to school, and the conditions are frequently conducive to a new wave.
These factors, along with the new variant XEC, have fuelled the current wave in the UK.
XEC is spreading so fast it is thought to be close to being dominant in the UK, if it isn’t already.
By contrast, because flu and RSV are fundamentally much less contagious – and produce far fewer new variants that can evade immunity – they need more extreme conditions to cause a wave.
There is also much more built-up immunity against these viruses, which have been round far longer than Covid.
As such, those viruses only really get going in November, peaking around Christmas and falling off sharply in January.
So what can be done to reduce the number of waves – and the size of those waves when they do hit?
“We can either increase our immunity by making better vaccines or increasing our vaccine coverage or we can slow the virus down with interventions, such as improving indoor air quality. But we’re not doing either of those things,” said Professor Griffin.
As a result, there is no sign of Covid becoming seasonal, he said, adding he would be “pleasantly surprised” if that happened this decade.
“People often talk about viruses running out of evolutionary space, so it can’t keep changing. But the trouble with this one is that it doesn’t seem to have reached anything near that limit and it seems to have this huge capacity to change and keep changing.
Like different branches of a very large tree,” he said.
“If it does become seasonal, it will take a long time based on the current rates. Whether it’s a disease of consequence as much in several years’ time – it hopefully won’t be.”
Professor Christina Pagel, of University College London, said: “Many people assumed it would become seasonal eventually, but the timescale for that could be years or decades.
“Covid is still intrinsically infectious enough and mutating fast enough and our immunity to infection wanes fast enough, that it can sustain waves throughout the year.
“I have no idea how long that will continue. Better vaccines, such as the new nasal ones being developed, might well help but who knows when they will become readily available?” said professor Pagel.
Professor Sheena Cruickshank, of Manchester University, said: “I would really like to see investment in vaccine technologies that may give more durable protection and that were offered more widely.
“Ideally we would be looking to develop or acquire more durable vaccines that confer longer-lasting immunity – such as nasal vaccines or multivariant universal vaccines that may be more resilient against the ever-evolving virus.”
Having said that, Professor Cruikshank says a significant uptake of vaccines in the UK could only go so far towards reducing the number of waves “as this is an infection that is circulating across the world”.
“So even if we vaccinated everyone in the UK there is still potential for the virus to evolve elsewhere,” she says.
