Pandemic payback: The real reason you have an everlasting cold

"We’re all playing immunity catch-up - or maybe that should be 'catch everything'." Photo / Jessica Peterson

EXPLAINER:

Most people never forget their first brush with the flu. I haven’t. Aged eight, I spent a week in bed, head pounding, feverish and shaking, and weak at the knees if I tried to get up. Luckily, this year, probably thanks to immunity from vaccines and previous infections, I dodged that bullet. Or did I?

Just before last Christmas, thousands of people in the UK went down with a nasty dose of flu. In our lab at the University of Cambridge, one in four of the samples we were testing from people with symptoms were positive for flu. That rate is way up on what we would consider a “normal year”. It’s also in stark contrast to the previous two flu seasons when we charted almost no cases at all. In fact, this year’s graph looks like Everest compared with a foothill the previous year.

Rates of flu are now in retreat, and Covid-19 too, but the reason they were so high is almost certainly a reflection on what nearly three years of social isolation has done to our population immunity. It also explains why, even for presumed “flu-dodgers” like me, most of us have felt like we’ve been incessantly unwell for months.

Flu is our virological canary in the coalmine. It’s one of the diseases we track and measure closely. A global network of laboratories collaborates to track the evolution and spread of the infection, and share data to inform how vaccines are updated so scientists can keep pace with what the virus is doing.

As such, a huge surge in flu cases also points to a likely huge surge in everything else we can catch, because just as birds of a feather flock together, respiratory viruses like influenza, coronaviruses, and rhinoviruses - which are all highly infectious and cause debilitating “common colds” - do too. There are hundreds of these viruses documented, which is sufficient for the average person to keep on catching new ones at the rate of two or three per year - for every year of their life - and still not run out of viruses to catch. That’s the status quo.

Every time we pop to the shops, go to a festival, or share a bus with an irritating sniffer, we’re also running into old viral foes we’ve already fought and defeated. Photo / Tania Whyte

But what’s going on in the background is that, normally, every time we pop to the shops, sit in the cinema, or share a train carriage with an irritating sniffer, we’re also running into old viral foes we’ve already fought and defeated. The encounter acts as an immune booster, strengthening our defences so we remain protected against the arsenal of agents we’ve already met in our lives.

The same happens with flu, which is why I cannot be absolutely sure I didn’t catch it this year. I may well have run into it, and even had it very mildly, but an existing well-prepared immune response will have controlled it promptly and restored me to good health. We’ve known this happens for a while. Doctors have surveyed patients, asking if they’ve had the flu recently. Testing those who answer “no” finds that a substantial proportion - based on their immune responses - have probably actually been infected recently. But because their immune system was already partially primed from previous flu encounters, they didn’t get severe symptoms and hadn’t realised that flu had come calling. The same thing almost certainly happens with other cough and cold viruses.

But when Covid-19 hit, the country locked down and international travel ground to a halt, so we also interrupted the spread of the majority of other seasonal infections into and around the country, including flu and colds. Many celebrated being cold-free for the longest period they could recall. However, the downside was this also meant that the natural infection process keeping our collective immune responses on their toes went into reverse, and we have developed a degree of immune amnesia for many of the common infections we’ve spent a lifetime learning to fight off.

The consequence of this immunity deficit is that it’s now payback time. Many of those things we would normally have batted away without an immunological second glance we’re now succumbing to again, in a low-grade way. The result is a relentless succession of infections, sometimes with multiple viruses at once. Although these can be mild and vague, nevertheless owing to sheer numbers these are laying us low and leading us to conclude we’ve had the same cough for months. Many people feel like they have long Covid, even if they haven’t. It’s a similar thing though: a succession of post-viral syndromes that rob you of energy and leave you feeling exhausted.

Now we’re all playing immunity catch-up - or maybe that should be “catch everything” - as we re-educate our immune systems to remember how to recognise the germs we once neutralised with ease. The bad news is that now norovirus, the winter vomiting bug, is back too [in the UK] with rates 30 per cent above normal. The good news is this immunity debt is probably one we’ll pay off in a year or two.

• Dr Chris Smith is a consultant virologist and lecturer at Cambridge University