TL;DR: There is no “immunity debt” for individuals. But *populations* can experience a “catch-up” period after a period of low virus transmission, which is what seems to be happening in China right now.
Recent reports of a high number of cases of childhood pneumonia in China definitely made epidemiologists a tad nervous. But so far, there is no evidence that these cases reflect a novel pathogen. Instead, it likely reflects a “catch-up” period for young people in China who were not as exposed to common viruses like flu and RSV in the past couple of years.
If you recall, last winter looked a lot like this in the US, with surging hospitalizations of kids for flu and RSV. There was also internet chatter that high levels of non-COVID respiratory infections were due to an “immunity debt” from reduced transmission during periods of COVID precautions. While many were referring to the population-level phenomenon, some were suggesting that people’s immune systems have gotten “weaker” for not having been exposed to these viruses. There is no truth to this idea—see our previous post here.
The fewer pathogen exposures over a lifetime, the better. (Just ask previous generations who suffered through measles, cholera, polio, smallpox, etc). For kids to develop a healthy immune system, they need exposure to “friendly” microbes in the natural environment (e.g. dirt).
Image credit: xkcd comics
For *populations*, it’s true that we are likely seeing some catch-up from previous low transmission. “Debt” is a bad choice of words, as there is no fixed number of infections we “owe” that must be paid back. Viral dynamics are driven by many factors, and it’s unlikely the flu, RSV, adenovirus, and other infections that were avoided during COVID would be canceled out by a “catch-up” period.
COVID precautions disrupted normal patterns of circulation of many pathogens, including flu and RSV. This means more people, especially young kids, are in the “susceptible” pool because they were not recently exposed to these bugs. It’s not surprising to see temporarily higher than average levels of these viruses as social interactions return to normal.
This does not mean that the overall benefits of reduced circulation of these viruses during COVID was canceled out now. We saw much reduced hospitalization due to things like asthma and COPD during COVID when common respiratory viruses were not circulating. Reducing baseline circulation of these viruses protects vulnerable people, it doesn’t create a “debt” that must be repaid in full to some virus loan shark.
The real problem with the use of “immunity debt” term is that it mixes up the question of population immunity (trivially true) versus effects on individual immunity (not true). The term did not exist in scientific literature prior to 2021, and the lack of precision in its use has led to a lot of confusion. The WHO is now referring to this dynamic in China as an “immunity gap,” which we like slightly better. Still, it’s important to remember this refers to a gap at the population level due to more susceptibles, not weakened immunity for individuals.
Around the world, we are playing some “catch-up” with viral transmission we missed in the past couple of years. This seems to be what is going on with childhood infections in China right now. The net benefits of having missed those infections are likely still high.
Our immune systems are not weakened through lack of viral exposures. The fewer viral infections the better. In fact, infection with measles actually “erases” immunity to other illnesses!
Stay tuned here for the science… and try to tune out the noise!