I think you're talking about another kind of seasonality. The common cold and influenza are seasonal in the sense that most infections occur during some time i

I think you're talking about another kind of seasonality. The common cold and influenza are seasonal in the sense that most infections occur during some time in the colder months. The researchers were trying to figure out how long-lived immunity to other coronaviruses is. Their question was not about the severity of cases, but whether a reinfection is possible. They found that in most cases, levels of antibodies against 4 common coronaviruses spiked about a year after the previous infection. That could either just mean that the patients still had immunity and their immune system was able to quickly produce more antibodies when they came in contact with the virus again. But it could also indicate a reinfection. In that case, symptoms would be observable. As symptoms were not recorded, we are unable to tell which case occurred more often.