Seroprevalence against COVID-19 and follow-up of suspected cases in primary health care in Spain

Background During the coronavirus disease 2019 (COVID-19) pandemic little information has been available about patients with mild or moderate symptoms attended and followed in the primary care setting, most of whom had an unknown status for the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Objectives We aim to measure the seroprevalence of antibodies against SARS-CoV-2 infection in a community sample of asymptomatic individuals and among symptomatic patients (without confirmed diagnosis) followed in a primary care setting. As a secondary objective, we estimated the proportions of symptomatic patients seeing at an emergency department (ED), hospitalized or dying, and identified the most important clinical symptoms associated with a positive infection. Methods From April 21 to April 24 2020, 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 (study population 1). From April 29 to May 5 2020, we also invited all the patients that had been followed by general practitioners (GPs) (study population 2). We used for both populations COVID-19 Rapid lateral flow immunoassay which qualitatively assesses the presence of patient-generated IgG and IgM in approximately 10-15 minutes. The prevalence (95% confidence intervals [CI]) of infection (past and current) was defined as the proportion of individuals with antibody seropositivity. Odds ratios (ORs) for a positive test result were estimated using logistic regression analysis. Results Three hundred and eleven asymptomatic individuals from the randomly selected sample accepted to participate in the study. The overall mean age was 43.7 years (SD 21.79, range 1-94) and 55% were women. Seventeen individuals were seropositive for IgM and/or IgG, resulting an overall prevalence of 5,47% (95% CI, 3.44-8.58). Six-hundred and thirty-four symptomatic patients were followed by GPs. The overall mean age was 46.97 years (SD 20.05, range 0-92) and 57.73% were women. Of these, 244 patients (38.49%) were seropositive for IgM and/or IgG. During the follow-up period, 27.13% of symptomatic patients attended the ED, 11.83% were hospitalized and about 2% died. Results of the multivariate logistic regression analysis showed that the OR for a positive test was significantly increased in patients who had fever (>38°C), ageusia and contact with a patient diagnosed with COVID-19. Conclusions The seroprevalence of antibodies against SARS-CoV-2 among asymptomatic individuals in the general population was lower than expected. Approximately 40% of the symptomatic patients followed by GPs during the peak months of the pandemic in Barcelona, were positive. Fever (>38°C), anosmia, ageusia and contact with a patient diagnosed with COVID-19 were associated with a positive test result.