Why does it feel like everyone is sick at the moment? - Gavi, the Vaccine Alliance

It was the bout of norovirus that nearly made me lose my sanity, coming as it did on the heels of weeks of severe respiratory illness. After developing pneumonia in mid-January, I tried to isolate myself from my daughter when she then caught a chest infection that sounded like marbles were rattling in her lungs, but small children tend to cling, koala-like, to their parents when they are sick. As my own immune system was already battered, I caught her infection too. The subsequent gastrointestinal bug was the cherry on this particularly horrendous cake.

There is a danger that "the immunity debt theory is being used to spread misinformation and promote a dangerous theory that natural infections are better than vaccines."

Winter sniffles are not an uncommon seasonal occurrence, and they are a regular feature in our home as I'm asthmatic and have a homemade germ factory with a primary school age kid. But this recent bout of illness has been unusually brutal. Despite my asthma, I've never been sick enough to require high-dose corticosteroids like prednisone. This time, I was. My daughter ended up having to take them as well.

What has been striking in the past few months is how many friends and family members have had respiratory illnesses lasting weeks, not days. Admittedly this is anecdotal, but it seems borne out by higher levels of respiratory infection than we've seen in previous winters. Health experts have warned of a 'tripledemic' of COVID-19, influenza and respiratory syncytial virus (RSV) that is fuelling pneumonia cases.

Worldwide, RSV is the most common cause of inflammation of the lungs (pneumonia) or the airways (bronchiolitis) in infants, resulting in around 3.6 million hospital admissions and killing more than 100,000 children under five each year. After malaria, it is the biggest killer of children under one. In December 2022, countries including France, Ireland, Spain, Sweden and the US detected an unusually early uptick in infection, with increased paediatric hospital admissions.

Cases of streptococcus A bacterial infection and associated scarlet fever have been unusually high in the UK, although they are in line with the last comparable high season of 2017-18.

Meanwhile flu and and COVID-19 are high too. In the US, the Centers for Disease Control and Prevention (CDC) reported that flu cases had spiked unusually early, with November 2022 seeing the highest flu cases for that time of year in a decade. Descendants of the COVID-19 Omicron variant continue to circulate, with the subvariant XBB.1.5 that has spread worldwide the most transmissible form of the virus yet. It is also more easily able to evade vaccines.

Immunity debt is an appealing yet unhelpful idea

One explanation that has been doing the rounds is the idea of an 'immunity debt' – the concept that as many of us have spent the previous couple of winters socially distancing or in lockdown, reduced contact with people has left our immune systems vulnerable because it hasn't been primed by natural infection.

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The idea is appealing on the face of it. But there is a difference between falling sick because of reduced exposure, and falling sick due to a weak immune system damaged by lack of exposure. The increased isolation over the previous couple of winters might mean a "greater naïve population" could explain a few infections say immunologists like Matthew Miller, at McMaster University in Hamilton, Canada, but not the idea that our immune systems have been damaged in some way because of being overly isolated.

It seems clear that on closer inspection, the idea of an immunity debt doesn't hold weight.

For one thing, immunity from past infections isn't necessarily long-lasting; this is why flu vaccines are reformulated every single year, as the influenza virus has the ability to rapidly evolve to evade our immune systems. Immunity from rhinoviruses – the number one cause of the common cold – is not very durable either. Even COVID-19 vaccines trigger strong immunity that wears off after a few months, requiring a booster.

The idea of an immunity debt also suggests that protecting ourselves from infection then requires a debt to be paid in the form of a weakened immune system, which is a dangerous idea. So says Peter Openshaw, a respiratory doctor and immunologist who studies RSV and flu at Imperial College London: "This would not be a good message for public health: we would still have open sewers and be drinking from water contaminated with cholera if this idea were followed to its logical conclusion."

There is a danger that "the immunity debt theory is being used to spread misinformation and promote a dangerous theory that natural infections are better than vaccines, and to rewrite the narrative that the pandemic response of social distancing, use of masks and rolling out vaccines was harmful and we are now paying the price – yet neither of these are based in fact," says Gustavo Correa, Senior Manager for Data Systems and Information at Gavi.

The safest way to prime our immune systems and protect us from disease is through vaccination rather than taking our chances with nature.

In fact, says Correa, even though we are seeing more RSV infections now, RSV infections in the first six months of life are much more dangerous. The pandemic response likely prevented many child deaths by RSV on that age group across the last few years. The same is also true for flu and COVID-19 deaths across different age groups

Preventing infection is always better than trying to elicit immunity from natural exposure, because it is often impossible to predict who will react badly to a virus if they are left open to infection. Many people hospitalised with SARS-CoV-2 were previously healthy, as are many of the children hospitalised because of RSV. "Preventing cases of flu and COVID-19 will also reduce the burden on health systems whose capacity has been overloaded in this winter wave of illness," adds Correa.

Common sense measures are important

"RSV spreads much more through contaminated surfaces," says Dr Scott Roberts, a Yale Medicine infectious diseases specialist. "A kid rubs snot on their hands and puts the hand on someone else, and then that kid puts their hand in their mouth, and they can be infected." Flu spreads in a similar way, which means that handwashing and keeping surfaces clean is especially important.

Measures that worked well in the pandemic continue to hold value says Michael Levin, professor of paediatrics and international child health at Imperial College London.

Even though mask-wearing on public transport is rarer than it was a year or two ago, Levin says, "It seems absolutely insane to me that we should get on a packed tube at rush hour and there will be people sneezing and coughing and not wearing a mask to protect others."

Protecting ourselves via vaccination

Some have suggested that COVID-19 may have weakened our immune systems, but there is no evidence for this so far.

Getting fixated on whether our immune systems have been ravaged by COVID-19 or weakened from a lack of exposure to pathogens is detracting from an obvious point, say experts like Openshaw, which is that we should be doing everything we can to protect ourselves through immunisation. "I can't think of an example of a viral infection being better than getting vaccinated," Openshaw says.

Although there are many unknowns as to why respiratory illnesses seem to be spiking this winter, what is certain is that rates of vaccination against many viruses are lower than they should be, and that is leaving us unnecessarily unprotected.

According to the CDC, In the US, only 16% of those eligible for a COVID-19 booster have taken it, even though it is better targeted to the Omicron variant than the original vaccines. Similarly, about 40% of adults have been vaccinated against flu. It is safe to have both the COVID-19 booster and the flu vaccine at the same time.

Rates of routine immunisation against major childhood diseases including measles, rubella and polio are also down in several countries.

In the US, routine vaccines among kindergarten students fell from 95% to 94% during the 2020-21 school year, and dropped further to 93% in the 2021-22 school year. School shutdowns and mobility restrictions meant vaccination programmes temporarily paused in 2020 and part of 2021, but continued disruptions to schooling and healthcare provision has meant several children are still behind.

In some cases, uncertainty around vaccination caused by misinformation may have triggered hesitancy in parents. A December 2022 survey published by the Kaiser Family Foundation found that more than one in three American parents said vaccinating children against measles, mumps, and rubella shouldn't be a requirement for them to attend public schools.

The good news is that there could soon also be a vaccine for RSV, as several pharma companies including Moderna, GSK and Pfizer have announced successful trials.

When a vaccine against RSV does eventually roll out, it will be critical that we remember that the safest way to prime our immune systems and protect us from disease is through vaccination rather than taking our chances with nature.