Mystery surrounds those who get infected by SARS-CoV-2 but never develop any symp­toms. How common are they in the population? Can they infect others? For how long? Do they develop immunity? Answers are starting to emerge.

Early in February, Chinese researchers began a detailed study of 37 asymptomatic people who had tested positive (via RT–PCR assay) for Covid-19. Because of China’s policy of iso­lating known or suspected cases, they were able to study these asymptomatic people in a hospital setting. This cohort was matched to 37 patients displaying Covid-19 symptoms, and a further 37 people outside the hospital who had not contracted the virus.

The main findings of the study, published last week in Nature Medicine, can be summarized as follows: the asymp­tomatic shed virus for a longer time than those with symptoms (19 days vs. 14 on average). Their immune response overall was weaker. While the asymptomatic exhibited antibodies to the disease, their levels dropped off more quickly after recovery.

This seems paradoxical to me. If someone gets infected but never shows symptoms, common sense would suggest that the person’s immune response to the virus was more effective than the norm, not weaker.

It is worrisome that any immunity from an asymptomatic infection looks like it wears off more quickly than that conferred by a more normal course of the disease. Two weeks into recovery, fewer asymptomatic patients (62% of them) showed short-term antibodies compared to the symptomatic (78%). And after eight weeks, the asymptomatic were more likely than their peers to demonstrate a drop in longer-term antibodies, 81% of them compared to 62% of those with symptoms.

The study was not able to shed any light on the questions of how infective the asymptomatic may be, or how long any contagiousness lasts. By design the study also had nothing to say about the fraction of the asymptomatic in the universe of Covid-19 infections.

One big question I am left with after reading this paper: Young children are often asymptomatic, or at worst develop mild symptoms that clear up in a few days (though a tiny fraction get a bad case). Are the reasons for this related to whatever drove the course of the asymptomatic in the Chinese study, or is it a matter of differences between immature and experienced immune systems?