This week a study from the UK caught my eye. Let us start by imagining that we take 100 people at random, put them in a school hall (why school hall? See below…) and send in a raging COVID-19 case to cough, sneeze, and splutter all over them. How many of those 100 will get infected and what do we mean by infected? For purposes of this thought-experiment, I will define infected as shedding enough virus to be detectable by the nasal swab test. You might imagine that all 100 would get infected, and that would inform your thinking about what is required for herd immunity -- public health officials talk about 60-70% of the population would have to get COVID-19 for there to be effective herd immunity assuming 100% susceptibility. However, there is plenty of anecdotal evidence that not everyone who is exposed to COVID-19 contracts it.

Researchers in the UK wondered if some people might already be immune to COVID-19, courtesy exposure to one, or more, of the four common corona viruses. There are a total seven known corona viruses which infect humans, four are no more serious than a common cold, the fifth is the original Sars-CoV-1, the sixth is MERS, and the seventh is making the rounds now. The researchers looked at antibody activity (not T-cell activity, which would be something else to consider) in both people who had had COVID-19 and people who had not had it. The first group provided validation that their antibody tests were working and shed light onto what parts of the virus the antibodies were attaching to (I’ll pass over discussion of those findings). The second group would help answer the question about how many people are already immune to COVID-19.

Amongst adults, it seems that the number of people with pre-existing immunity is north of 10%, and this immunity appears to be due to exposure to one, or more, of the four common corona viruses. The eye-popping finding was the percentage of school age kids with pre-existing immunity, courtesy the common corona viruses: it is over 60%. Let me add that the distribution of people with “recent exposure to one of the four common corona viruses” (meaning in the last few months), is highest in school age children and declines with age, and exposure is highest in winter months. There has been much speculation as to why younger folks don’t seem to be bothered by COVID-19, and this study may have provided the first rigorous reason: youngsters don’t get symptomatic COVID-19, because they are likely to have had recent exposure to a common corona virus.

If exposure to the four common corona viruses is indeed protective, then it has a bearing on the opening schools for the Fall and Winter terms. If we institute measures to limit transmission of COVID-19 amongst school children we will also be limiting the spread of the common corona viruses, and their protective effect. This provides an opening for the law of unintended consequences: we would see a rise in the number of severe COVID-19 cases amongst kids, and their pandemic “honeymoon”, if you will, would be over. Ironically, teachers who fear picking up COVID-19 from school kids, would also lose out on the protective effect of being exposed to the common corona viruses from their favorite little Petri dishes. I should add that while this scenario has not been “proven”, I suspect that it is going to be tested in the coming months.