Many blame the perceived increase in illness on lockdowns, and the idea that children have somehow accrued a so-called ‘immunity debt’ by not being exposed to as many pathogens as usual.

As reassuring as this instinctive hunch might seem, the evidence is starting to tell us something else.

We’ve written before about the potential immune harms caused by COVID-19. Despite the real risks it poses to children’s health as a leading cause of infectious disease mortality1, some commentators still believe infection with SARS-CoV-2 is a net benefit because it is training children’s immune systems to recognize the virus, which will in theory give them greater ability to avoid the serious consequences of infection later in life. This is the so-called “novelty is severity” argument.

But is it possible COVID-19 is adversely affecting children’s immune systems and making them less able to deal with other infections? Some immunologists are concerned that SARS-CoV-2 may indeed be harming children’s immune systems for an unknown period.

A large study from Israel suggested children were at higher risk of strep tonsillitis up to a year after COVID-19, when a matched analysis was done between children with and without COVID-192.

Another large study from the US recently showed a higher risk of respiratory syncytial virus (RSV) infections and bronchiolitis in late 2022, after a COVID-19 infection, in a matched cohort of children who had COVID-19 pre-August 2022. This suggests prior COVID-19 infection may have contributed to the large wave of RSV in children in 20223.

This study included 228,940 children aged 0–5 years who had medical encounters with healthcare in October 2022 and had no prior attendance to the clinic for RSV. Of these 14,493 children had contracted COVID-19 prior to August 2022.

Those who had documented COVID-19 at any point before August 2022, were compared with those who didn't have COVID-19 prior to August 2022. Their attendance to healthcare for RSV and bronchiolitis of unknown cause was measured from Oct-Dec 2022 in health records.

In studies like this, matching both cohorts is essential. It's no surprise that children at higher risk for COVID-19 would also be at higher risk for RSV, because of factors associated with exposure, such as crowding, socio-economic status etc., so it is important to correct for these. The study matched both groups for socio-economic status, previous health conditions, age, sex, ethnicity- and found a 40% higher risk for RSV infections in 0-1 yr olds and 0-5 yr olds who had been exposed to COVID-19.

The authors of the study suggest that it is possible that increased susceptibility to respiratory infections also contributed to the large wave of RSV infections seen in the US, which translated not just into increased cases, but also into very high levels of hospitalisations for RSV.

While this study suggests a link between prior COVID-19 infection and risk of RSV, it's not possible to definitively conclude that COVID-19 caused the increase in RSV, because this is an observational study. It's possible that despite matching extensively for external factors that make those who are at risk for COVID-19 also at risk for RSV (socio-economic status, asthma, etc), there is what is known as 'residual confounding'- so all measures may not be fully corrected for.

But it suggests there is a link, and more work must be done to understand whether COVID-19 is harming children’s immune systems in a way that increases susceptibility to infections like strep or RSV. This would not be the first time a virus has been found to impact the immune system. Measles is known to cause immune amnesia4,5.

While scientists work to understand the impacts of a virus that has been with us less than 4 years, we recommend doing whatever is reasonably possible to avoid infection with COVID-19. You can find advice on how to reduce your risk of infection here.

This piece was inspired by a Twitter thread by Dr Deepti Gurdasani and sections have been used with her permission. Her original thread can be found here.