According to a recent study from the Karolinska Institute in Sweden, T-Cell reactivity against SARS-CoV-2 increased from 28% in 2019 preserved blood samples to 46% in 2020 blood samples from healthy donors. That would suggest that, unless there has been a massive increase in the prevalence of some other virus that induces cross-immunity to SARS-CoV-2 around the same time, we can infer that 46 - 28 = 18 + (28 x 0.18) = 23.04% of the population has been exposed to SARS-CoV-2 in Sweden this year, plus whoever has been diagnosed positive in the antigen tests. I think antibody tests alone suggested about half that number, suggesting the importance of gauging cellular immunity.
On the other news, antibody data from Daegu in South Korea suggests the true prevalence of the virus there was 27 times more than what has been oficially diagnosed (around 6.2% of the population, compared to 0.033% having antibodies in the rest of South Korea). The relatively low IFR there (0.11% compared to 0.68% in the rest of South Korea, based on previous testing that excluded Daegu) suggests it was mainly younger people that drove the initial Daegu outbreak in February, compared to other places in Europe etc. where it started in hospitals and retirement homes. Of course, as shown above, real prevalence could be significantly higher.
