Why scientists are rethinking the immune effects of SARS-CoV-2
(BMJ)
Mycoplasma pneumoniae is a bacterial infection not known to cause widespread hospital admissions. “I can count on my two hands the number of times I’d ever seen
mycoplasma pneumoniae before 2023,” says Samira Jeimy, clinical immunologist at the University of Western Ontario. “All of a sudden I feel like everybody has it.”
1
Over the past three years similar reports have circulated of rising bacterial infections, flare-ups of old viruses becoming more common, and children landing in hospital with diseases not usually seen in young, healthy people. One explanation offered by public health leaders has been “immunity debt”
2—the idea that precautions taken in the covid pandemic suppressed routine exposures to circulating pathogens, leaving people more vulnerable to them when restrictions were lifted.
[...]
Immunity debt—or disruption?
Malgorzata Gasperowicz, a Calgary based developmental biologist, says that if immunity debt fully explained rising infection counts we’d expect to see a uniform rebound across all pathogens. But we don’t, she says.
For instance, a 2024 study of more than 4000 viral cases from Ontario, Canada,
4 found higher rates of bacterial infections in people recovering from covid-19 than in those recovering from influenza or RSV—although study groups weren’t perfectly matched by age or clinical setting, limiting direct comparisons.
Jeimy says that many infants and toddlers admitted to hospital with rare infections since 2022
5 weren’t yet born when pandemic restrictions were in place, and they therefore couldn’t be experiencing immunity debt. They were, however, likely exposed to SARS-CoV-2.
Wolfgang Leitner, chief of the Innate Immunity Section at the US National Institute of Allergy and Infectious Diseases (NIAID), speculates that covid-19 may somehow impair the immune system’s “memory” of past infections, potentially making even healthy people more vulnerable to future pathogens. He wonders whether the virus leaves lasting scars on the immune system’s T cell defences. “But that’s just (my) hypothesis,” he emphasises in an email.
Immunity reset?
SARS-CoV-2 is linked to “an unusually high level of ‘indiscriminate’ killing of T cells,”
6 says Leitner, adding that this observation is “reminiscent of” measles, which can cause immune amnesia by depleting memory B cells (a different type of immune cell), leaving people vulnerable to pathogens they were previously immune to.
7
This concept of immune “reset” after infections isn’t new. A hallmark of this phenomenon is the reactivation of dormant viruses, which re-emerge while the immune system is in a weakened state. Reactivation of viruses, including Epstein-Barr virus (EBV) and varicella zoster virus (VZV), has been commonly observed after covid-19.
[...]
“Immunity debt,” a theory to explain the global surge in non-covid infections since pandemic restrictions were lifted, is increasingly being challenged by emerging evidence. Nick Tsergas reports Mycoplasma pneumoniae is a bacterial infection not known to cause widespread hospital admissions. “I...
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