Community seroprevalence of COVID-19 in probable and possible cases at primary health care centres in Spain

Background: There is a scarcity of information about patients with mild or moderate symptoms during the coronavirus disease 2019 (COVID-19).This is especially true for those who attended and were followed up at primary care settings. Objectives: We aim to measure the seroprevalence of antibodies against SARS-CoV-2 infection in a community sample of possible cases and among probable cases followed in primary care. Methods: We selected a random sample of 600 individuals stratified by age groups from a total population of 19 899 individuals from a community area in Barcelona. We also invited all the patients that had been followed by General Practitioners (GPs). For both populations, we used COVID-19 rapid lateral flow immunoassays, which qualitatively assess the presence of patient-generated Immunoglobulins G (IgG) and Immunoglobulin M (IgM). Results: Three hundred and eleven asymptomatic individuals from the randomly selected sample participated in the study. The mean age was 43.7 years [standard deviation (SD) = 21.79] and 55% were women. Seventeen individuals were seropositive for IgM and/or IgG, resulting in an overall prevalence of 5.47% (95% confidence interval = 3.44-8.58). Six hundred and thirty-four symptomatic patients were followed up by GPs. The mean age was 46.97 years (SD = 20.05) and 57.73% were women. Of these, 244 patients (38.49%) were seropositive. Results of the multivariate logistic regression analysis showed that the odds ratio for a positive test was significantly increased in patients who had fever, ageusia and contact with a patient diagnosed with COVID-19. Conclusions: The seroprevalence of antibodies against SARS-CoV-2 among possible cases was lower than expected. Approximately, 40% of the symptomatic patients followed up by GPs during the peak months of the pandemic were positive.

Automated summary

This study aimed to measure the seroprevalence of antibodies against SARS-CoV-2 infection in a community sample of possible cases and among probable cases followed in primary care. The results showed that the seroprevalence of antibodies among possible cases was lower than expected, while approximately 40% of symptomatic patients followed up by GPs during the peak months of the pandemic were positive.