According to the results of the first Covid-19 National Serological Survey “Portugal registers a global seroprevalence of 2.9 percent of infection by the new coronavirus in its population [close to 300,000 people], with no significant differences found between regions and age groups”, said the National Health Institute Doctor Ricardo Jorge (INSA), promoter of the study.

Seroprevalence was higher in people who reported having had previous contact with a suspected or confirmed case of Covid-19 (22.3 percent versus 2.0 percent) and in those with symptoms compatible with the disease - fever, chills, asthenia, odynophagia, cough, dyspnoea, headache, nausea / vomiting and diarrhoea - (6.5 percent versus 2.0 percent).

About 44 percent of people with specific antibodies against the new coronavirus did not report any previous symptoms of Covid-19.
The data shows that the differences between the percentage of the population with Covid-19 antibodies and the number of reported cases of infection are due to difficulties found in surveillance systems in capturing mild or asymptomatic cases of the disease.

The survey aimed to “characterise the distribution of specific antibodies against SARS-CoV-2”, determine the extent of infection in the population resident in Portugal, determine and compare seroprevalence by age group and by Health Region and the numbef of asymptomatic infections.

Regarding the distribution by sex, the study indicates that the estimated seroprevalence was higher in men (4.1 percent) than in women (1.8 percent).

Analysing the ages, similar values were found for the age groups under study, varying between 2.2 percent (from 10 to 19 years old) and 3.2 percent (from 40 to 59 years old).

Among the different Health Regions, the percentage of the population with antibodies to SARS-Cov-2, which causes Covid-19 disease, varied between 1.2 percent in Alentejo and 3.5 percent in Lisbon and Vale do Tejo, “although without statistical significance ”.

For the coordinator of the survey, Ana Paula Rodrigues, the results provide “advice for the maintenance of individual and collective protection recommendations for all individuals, regardless of the respective level of specific antibodies against SARS-CoV-2”.

They also show “the need to monitor the evolution of the seroprevalence of these antibodies in the population, according to the evolution of the epidemic in Portugal”, said Ana Paula Rodrigues.

The fieldwork of the survey took place between 21 May and 8 July, with 2,301 participants over the age of one year, spread across all health regions of the continent, Azores and Madeira.

The results also reveal a global seroprevalence according to values obtained in other population-based and nationwide seroepidemiological studies carried out in other countries.

The tests were carried out in 96 clinical analysis laboratories and 18 hospitals of the National Health Service (SNS) that are partners in the National Serological Survey.