After two long winters of covid-19, now the northern hemisphere is enduring a difficult season of multiple infectious illnesses. Other respiratory infections, like influenza and those caused by respiratory syncytial virus (RSV), are running rampant, particularly in children 鈥� leading to waves of hospitalisations and a number of deaths. While it is normal for respiratory infections to spike in the coldest months, the timing of the waves and the number of hospitalisations both in children and the wider population this winter are unusual.
What is going on? A commonly offered explanation is that we are seeing the after-effects of covid-19 mitigation measures, such as lockdowns, remote learning and limits on social mixing. The suggestion is that, in 2020 and 2021, children were less exposed to infections because they didn鈥檛 see their peers as much as they normally would, leading to a backlog of infections taking place now.
This is undoubtedly a big factor, but it isn鈥檛 the whole story. Some of the strains of pathogen currently circulating cause unusually severe disease in children. And crucially, pathogens 鈥� including the coronavirus 鈥� are probably interacting with each other, affecting both the timing and severity of the infection waves.
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Childhood respiratory infections
What we鈥檙e seeing in children is a result of waves of infections passing through the whole population. While covid-19 has far from gone away 鈥� hospitalisations are currently increasing in both and the , for example 鈥� a host of other respiratory infections are now surging through. They always do at this time of year, but the current situation is unusually bad.
Take influenza. The winter wave started early: total population hospitalisations for influenza in the US have been and 30 children are reported to have died. Meanwhile, Canada has seen children being hospitalised for influenza for this time of year: at least alone. In the UK, more people were admitted to hospital for flu in the week ending 11 December than . While flu can cause hospitalisations and deaths across the whole population, children under the age of 5 are among those that are at a higher risk of developing complications as a result of flu.
RSV, which regularly causes cold-like illnesses in young children, has also been surging in northern hemisphere countries over the past two months, threatening to overwhelm hospitals. While most cases are mild, it can turn deadly, particularly in children under 5, and, prior to the covid-19 pandemic, it was estimated to cause in this age group in the US.
There have also been numerous cases of streptococcus A (strep A), a bacterial throat infection that sometimes enters the bloodstream and causes severe illness, known as invasive group A streptococcus (iGAS) disease. On 15 December, the UK Health Security Agency reported that . One primary school (for children aged under 12) was for a 鈥渄eep clean鈥� after 40 per cent of children and 23 per cent of staff were absent due to a strep A outbreak.
France, Ireland, the Netherlands and Sweden have also seen increases in the number of cases of iGAS disease, as well as scarlet fever, which can be caused by strep A. For both iGAS disease and scarlet fever in these countries and the UK, the World Health Organization reports that it is who have been affected.
The result is a perfect storm of childhood infections. 鈥淚鈥檓 here in the Netherlands and RSV rates are through the roof,鈥� says of the Boston University School of Public Health in Massachusetts. His paediatrician wife, still in the US, has 鈥渘ever been busier in her life鈥�.
Why now?
In some cases, the currently circulating strains of the pathogens are more dangerous. Notably, almost all influenza viruses infecting people at the moment are . 鈥淲e know it鈥檚 more severe and can hit children and older people harder, and that鈥檚 exactly what we鈥檙e seeing,鈥� says at the Vaccine and Infectious Disease Organization in Saskatoon, Canada. The dominant strain of strep A .
However, the biggest factor underlying these waves of childhood illness may simply be that pathogens are playing catch up now that most covid-19 mitigation measures have ended and children are fully mixing again. 鈥淭he evidence is very, very strong that the lockdowns had a profound impact on transmission of childhood diseases, nearly all of which transmit through saliva and snot,鈥� says Gill.
His team tracked RSV deaths among babies and children who died in Lusaka, Zambia between mid-2017 and mid-2020, and found the virus present in. However, they found that rates of RSV mortality fell by about two-thirds in 2020 when Lusaka went into lockdown.
Similarly, a September study by the UK Health Security Agency found that RSV was in England the winter of 2020-21, with laboratory-confirmed cases falling 99.5 per cent. However, this was followed by an unprecedented surge in summer 2021, even though cases are normally rare at this time of year.
The phenomenon of diseases surging at unusual times has been widespread: in Australia, the 2020 RSV outbreaks , presumably having been delayed by covid-19 restrictions.
This emphatically doesn鈥檛 mean we would have been better off letting our kids get influenza and other respiratory infections in 2020, says Kelvin. 鈥淚nfluenza will cause immunosuppression and unfortunate long-term consequences such as asthma,鈥� she says. 鈥淲e want to decrease the number of times we get it, because each time we do, it causes pathology in our lungs.鈥� Kelvin notes that influenza vaccines provide similar levels of immunity without the harm caused by getting infected.
Although infections like influenza and RSV are commonplace, they are clearly far from harmless. Earlier this year, Gill and his colleagues estimated that 1 in 28 deaths in children aged 1 to 6 months in 2019 . Another group estimated that RSV born in high-income countries. The global burden of strep A is less well understood, but a study from June estimated that, every year, there are aged 5 to 14 鈥� so, even though the severe form is rare, there are enough infections to cause a significant death toll worldwide.
Viral interference
The final factor likely to be at work in the current surge of infections affecting both children and the wider population is that viruses interact with each other, a phenomenon called .
鈥淚f you鈥檙e infected with one virus, it often blocks another virus from being able to infect you at the same time,鈥� says Kelvin. This makes sense for the virus: it wants to make more copies of itself and having another virus muscle in would limit its ability to do so. 鈥淭hat鈥檚 why we see waves of certain viruses at certain times, because the virus is hitting a susceptible population and it鈥檚 getting them all at once, and then while it鈥檚 doing that, it鈥檚 blocking other viruses from hitting the same population,鈥� Kelvin says.
However, there is also a second process. Viruses suppress our immune systems and this effect can linger for weeks after a particular virus has left our bodies. As a result, a few weeks after an infection, we are liable to be infected by something else.
Both processes play out in a typical . There is usually a peak of cases of one form called influenza A, often in the middle of winter, followed by a peak of influenza B in late winter and early spring. comes in once influenza A isn鈥檛 there and takes advantage of people鈥檚 suppressed immune systems from previous infections.
The role of covid-19
But these dynamics are playing out differently this year because disease waves are starting at unusual times, partly because of the after-effects of covid-19 restrictions and also partly because , running its own viral interference. The influenza wave started early in Australia this year and then did the same in the northern hemisphere.
As for the unusually severe levels of disease among children and the wider population, Kelvin suspects repeated infections with SARS-CoV-2, the virus that causes covid-19, are a factor. 鈥淲e鈥檝e been getting hit over and over again with SARS-CoV-2, and now influenza,鈥� she says. Our bodies aren鈥檛 getting enough time to recover fully from one infection before the next one comes along.
It isn鈥檛 clear how long these unusual waves of infection will continue. 鈥淲hat I would imagine would happen is that these typical cyclic patterns that we鈥檝e seen for 100 years will probably reassert themselves,鈥� says Gill. Viruses like influenza and RSV will establish a new annual pattern. However, it might not be exactly the same as before, because the coronavirus will be there too. Kelvin says this year鈥檚 earlier start to the influenza wave might become the new norm, as influenza and covid-19 find an equilibrium.
The way to control the waves is regular vaccination, says Kelvin. Our immunity to covid-19 and influenza 鈥� whether by infection or immunisation 鈥� only lasts about six months, so the vaccines should be given at least once a year, particularly to the age groups most at risk. Meanwhile, multiple RSV vaccines are in development and some may be available in 2023, including one for children. This opens the possibility of vaccinating people against influenza, covid-19 and RSV every autumn. 鈥淚t鈥檚 a great strategy to protect people, especially children,鈥� says Kelvin.
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