This Icelandic study measured a variety of IgG and IgM antibodies to SARS-CoV-2 in six groups of Icelanders who had either never been tested or had tested negative and in two groups of Icelanders who had tested positive and had recovered. They weighted the sample by age and sex to estimate the number of Icelanders who had been infected and used that to estimate the infection fatality ratio (deaths/[symptomatic + asymptomatic positives]) and case fatality ratio (deaths/symptomatic positives). Among patients who had experienced a symptomatic infection, IgG antibody levels remained stable over a four-month period, which is encouraging in terms of at least medium-term immunity. Those who smoked and those taking anti-inflammatory medications had lower antibody levels, although it is not clear whether this difference is clinically significant. This was not the case for IgM and IgA antibodies, which is what you would expect. They also estimate that the infection fatality ratio was 0.3% (95% CI, 0.2% to 0.6%), and the case fatality ratio was 0.6% (95% CI, 0.3% to 1.0%). The infection fatality ratio was 0.1% (95% CI, 0.0% to 0.3%) in people 70 years or younger and 4.4% in those older than 70 years (95% CI, 1.9% to 8.4%).
Source: Gudbjartsson DF, Norddahl GL, Melsted P, et al. Humoral immune response to SARS-CoV-2 in Iceland [published online September 1, 2020]. N Engl J Med. 2020. DOI: 10.1056/NEJMoa2026116).