Although COVID infections are currently low or declining in most western countries except the UK, there is still a long way to go before the threat of the pandemic is over. A major concern this winter is if there is a resurgence of COVID with other respiratory diseases coming back strongly next to it – especially the flu.
In both the southern and northern hemispheres, influenza infections declined rapidly and largely disappeared in the pandemic. Social distance measures used to contain COVID were even more effective in reducing the spread of influenza. But unfortunately, that means we now have to be prepared for the flu to get particularly bad this year.
In some ways, immune responses to COVID and influenza are the same. A relatively recent infection or vaccination provides good protection against a subsequent infection, but soon this protection begins to wane. However, early re-infections are generally asymptomatic or relatively mild. However, the longer the distance between the generation of immunity and the re-infection, the more serious the subsequent infection is likely to be.
This is especially evident when looking at how the flu affects people living on remote islands. Because they can go for long periods without being exposed to the flu when they eventually encounter it, their mortality rate is higher. A study looking at people living in Tristan da Cunha, a group of isolated South Atlantic islands, found that it was only a few years between flu releases that increased the risk of the disease.
So the concern is that with COVID control measures that have almost completely limited people’s exposure to the flu over the last 18 months, natural immunity will have declined across the population. We have pretty much all lived on remote islands since the beginning of 2020 thanks to the barriers, travel restrictions and work from home that have been put in place.
Therefore, when the flu returns, it can affect more people and cause more serious illness, as we would normally see in a typical flu season. The same is likely to apply to other respiratory viruses. In fact, this may already be happening – recently there have been many anecdotal reports of people getting particularly severe colds in the UK.
However, it is not easy to predict when exactly the flu will return, nor how much worse or more common it will be. At present, the flu rate is still quite low in the UK, but this could change quite quickly if the virus starts to spread.
Fortunately, we have safe and effective flu vaccines that reduce both the risk of infection and serious illness. However, they are not as effective as most of the current COVID vaccines. Plus, how well they work varies from year to year. Influenza viruses mutate faster, meaning more strains end up circulating and changing each year. If what turns out to be the dominant viral strain every winter is not included in the vaccine, its effectiveness will be lower.
Recommendations for which viruses to include in the annual flu vaccines – created separately for the winters in the northern and southern hemispheres – are made by the World Health Organization, which estimates that the strains have circulated in advance. However, as influenza cases have been so low over the last 18 months, it is more difficult than usual to predict which viruses will be dominant this winter. So in addition to potentially being more susceptible to the flu, this year too there is a greater risk than usual of getting a vaccine that is not as effective as usual.
How this is combined with COVID
Even before the pandemic, winter flu added significant pressure to both GPs and hospitals each year. Dealing with it now, at a time when the healthcare system has many COVID patients on hospital beds, would be particularly difficult and would further intensify the pressure on the healthcare system. Ultimately, more strain on the NHS puts people’s health at risk.
But there is also an additional risk: co-infection. It is possible to have COVID at the same time as another bacterial, fungal or viral infection. In fact, a study looking at hospitalized COVID patients estimated that 19% also had another infection. It found that patients with co-infections were more likely to die.
Early in the pandemic, when influenza was still circulating, British researchers were able to compare the results of humans with COVID alone vs a coinfluenza co-infection. People with a co-infection were about twice as likely to be admitted to intensive care, twice as likely to require ventilation, and about twice as likely to die than those who just had COVID.
It is not possible to say whether we will see a major flu epidemic in the UK this year, but if not, there will almost certainly be one soon. And when the flu returns, it is likely to affect more people than in most pre-COVID years and cause more deaths than usual. The number can be large. In a bad winter, the flu kills over 20,000 people in England.
Because of the direct threat – and the increased pre-influenza will put on healthcare services that are likely to still struggle to cope with COVID – it is doubly important that people take the offer of flu and boost COVID vaccines if and when they are offered in the fall .