What is the Actual Prevalence of SARS-CoV-2 in any population and country?

An interesting study published in The Lancet by Marina Pollán, Beatriz Pérez-Gómez and others performed a nationwide population-based study to estimate the seroprevalence of SARS-CoV-2 infection in Spain at a national and regional level. 

They selected 35,883 households from municipal rolls and 75·1% of those contacted answered a questionnaire on history of symptoms compatible with COVID-19 and risk factors.  They received a point-of-care antibody test and if agreed, donated blood for additional testing.  From this technique the authors calculated a seroprevalence range maximising either specificity positive for both tests or sensitivity positive for either test. 

They found that the seroprevalence was 5% by the point-of-care test and 4.6% by immunoassay with a specificity–sensitivity range of 3.7% to 6.2% for either test positive.

There appeared to be no differences by sex but a lower seroprevalence in children younger than 10, less than 3.1% by the point-of-care test.

There was a substantial geographical variability with higher prevalence around Madrid, greater than 10% and lower in coastal areas, less than 3%. 

Seroprevalence among 195 participants with a positive PCR of more than 14 days before the study visit ranged from 87.6% with those who tested positive for both tests to 91.8% with either test positive.

In 7,273 individuals with anosmia or at least three symptoms, seroprevalence ranged from 15.3%. 

Around a third of seropositive participants were asymptomatic ranging from 21.9% to 35.8%. 

Only 19.5% of symptomatic participants who were seropositive by both the point-of-care test and immunoassay reported having a previous PCR test. 

The authors concluded that the majority of the Spanish population is seronegative to SARS-CoV-2 infection even in hotspot areas. 

Most PCR-confirmed cases had detectable antibodies, but a substantial proportion of patients with symptoms compatible with COVID-19 did not have a PCR test and at least a third of infections determined by serology were asymptomatic. 

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