Has Covid messed with our immune systems?
China, immune debt, and the latest immunology data story
Last week, a scare around an undiagnosed cluster of pediatric pneumonia in China rippled through social media. This was coupled with reports of overwhelmed pediatric hospitals in certain regions. The WHO confirmed this isn’t likely due to some novel pathogen but rather a resurgence of our “regular” viruses, like flu.
This has, nonetheless, reignited a discussion around “immunity debt” and whether Covid-19 infections make us immunocompromised.
In the first 2-3 years of the pandemic, RSV and flu, for example, were incredibly suppressed due to pandemic control measures. This led to a new phrase: “immunity debt” or “immunity gap.”
It’s centered around an increasing “susceptibility” pool, or how many people are vulnerable to infection. During the emergency, more and more people became susceptible due to a lack of infection (or maternal antibodies), susceptible people were born, and recovered people died. Then, when restrictions were lifted and the “normal” viruses returned, there was a HUGE susceptibility pool leading to many infections. This resulted in more infections than in the pre-pandemic seasons, which caused crowded hospitals in China, for example.
The U.S.—which is one year ahead of China in lifting restrictions—still has higher than “normal” RSV infections. It will likely take a few years to re-synchronize viral dynamics, but the dramatic effect is less apparent every subsequent year. Below is my best attempt at visualizing this phenomenon.
While the term might be new to the public, this phenomenon has been seen before. For example, when sanitation improved, polio initially got much worse.
Immunocompromised after Covid-19 infections?
There has been discussion around whether anything else could be causing this rise in pediatric infections, like: Could there be more viral infections because Covid-19 is doing something weird with our immune systems?
This does happen with other viruses. HIV and measles are notorious for it, for example.
However, the case for Covid-19 is weak thus far:
T cells. Early in the pandemic, scientists found that the T cells expressed many markers that implied exhaustion (or low T-cell count). After more inquiry, it was clear the actual behavior of the T cells was still very much functional. Another concern was a drop in the blood’s T cell levels. But low counts can be attributed to T cells doing their job: leaving the blood and going to the tissues to fight off infections. This can happen after any infection, even with the common cold.
B cells. One paper argues that Covid-19 infections might harm B cell responses (i.e., our antibody factories). This hasn’t been reproduced. Also, the high antibody levels we see after vaccination and infections show our antibody factories are still working.
Epidemiological RSV data. One study has shown that children are at increased risk for RSV infections after a positive Covid-19 test. While this concerns us parents, there were many limitations. For example, the Covid-19 negative patients were not tested for SARS-CoV-2 antibodies. If severe RSV and Covid-19 share risk factors in young kids, this could lead to artifactual findings exaggerating the risk.
We certainly need more studies. But there are specific situations that are genuinely concerning regarding immunocompromise after Covid-19 infections:
Severe Covid-19 disease can cause you to develop antibodies against certain messengers (called cytokines like interferon), which can block critical messages from being sent, drastically reducing your ability to respond to infections. This is seen after critical pneumonia, too. The good news is that, now, these severe cases of disease are more rare, and in most people, this issue fades over time.
Some niche immune cells seem to change a lot after Covid-19 infection, but they are relatively minor populations, and the consequences to our overall health from these changes aren’t entirely clear. (Also, these data are all pre-vaccine.)
Neither of these states is common enough in young children to explain current epidemiological trends.
This is not to say that Covid-19 is something that we can just ignore—it comes with plenty of risks unrelated to what it may or may not do to the immune system (such as blood clots), and many still suffer from long Covid-19 for reasons we don’t fully understand.
We will continue to see “immunity debt” play out on a population level. Keep up to date on vaccines and take reasonable precautions against respiratory viruses. This will help keep both healthcare systems and ourselves as healthy as possible.
Love, YLE and EN
“Your Local Epidemiologist (YLE)” is written by Dr. Katelyn Jetelina, MPH Ph.D.—an epidemiologist, wife, and mom of two little girls. During the day, she is a senior scientific consultant to several organizations, including the CDC. At night she writes this newsletter. Her main goal is to “translate” the ever-evolving public health world so that people will be well-equipped to make evidence-based decisions. This newsletter is free, thanks to the generous support of fellow YLE community members. To support this effort, subscribe below: