For several months, we have been inundated by reports summarizing incidence and mortality data for COVID-19, on both the national and global level. In a previous ProMED post I cautioned that “reported cases” cannot be equated with “total cases” without inclusion of individuals with asymptomatic of sub-clinical infection that do not seek medical care.  If a large segment of the population is found to be seropositive, we might conclude that the true case-fatality ratio of COVID-19 is lower than official data might suggest. A seroprevalence study reported this week seems to provide solid evidence that this is the case. 
A national survey of individuals identified IgG antibody toward SARS-CoV-2 in 5.0% of the general population of Spain. At the stated specificity of 100% and sensitivity of 98%, the true seropositivity may be estimated at 5.15%. Although factors which determine seroprevalence rates in Spain need not apply to other countries, the following chart extrapolates the potential impact of a 5.15% population seroprevalence on case-fatality data from high-incidence countries in the European Region, United States and China. 
The impact of future SARS-CoV-2 seroprevalence surveys will largely depend in the quality of the test itself, the duration of immunity and protective role of the antibody, possible emergence of newer strains of coronavirus, and other factors. In any event, additional seroprevalence data will play a key role in planning our response to this pandemic going forward.
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