Mild cases of COVID infections contribute to herd immunity.

https://www.nejm.org/doi/full/10.1056/NEJMp2026913?query=recirc_mostViewed_railB_article

While advancing a tentative hypothesis that widespread masking might reduce the severity of infections (due to smaller amounts of virus reaching our noses), these two experts in Epidemiology make a much more important point. They make it abundantly clear that even mild or asymptomatic SARS-CoV-2 infections almost certainly contribute to population-wide immunity ("herd immunity"):

"Reinfection with SARS-CoV-2 seems to be rare, despite more than 8 months of circulation worldwide . . . The scientific community has been clarifying for some time the . . . inadequacy of antibody-based studies to estimate the level of more durable T-cell and memory B-cell immunity to SARS-CoV-2. Promising data have been emerging in recent weeks suggesting that strong cell-mediated immunity results from even mild or asymptomatic SARS-CoV-2 infection, so any public health strategy that could reduce the severity of disease should increase population-wide immunity as well."

If these scientists were mistaken about substantial population-wide immunity being much more attainable than its critics claim, we could not possibly explain why supposed examples of reinfection are so rare if they exist at all (testing makes mistakes).

Without some degree of population-wide immunity from uncounted asymptomatic past cases, we could possibly not explain why so-called "second waves" are so rare if they exist at all.

Second waves are never seen in places like New York City or Sweden that suffered high death rates (and many unseen cases) in the past.

What the press loosely describes as "second" waves (the U.S. South in July, Dakotas in August, or Croatia today) were almost always relatively gentle first waves.

https://www.nytimes.com/interactive/2020/world/europe/france-coronavirus-cases.html

The trouble with using confirmed (tested) cases rather than deaths to define a second wave is that testing was very low in April and is often speeding-up now. Many countries, like many U.S. states (New York) had almost no idea how many cases they had during the March-April "first wave" because testing was mainly confined to obviously sick people in hospitals.

"France was late to increase its testing capacities compared to countries like Germany. The national public health authority has acknowledged that the reported number of test-confirmed cases was lower than the real number of cases."

Rather than calling this a second wave of infections it might be more accurate to call it a first wave of serious testing.