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Coronavirus COVID-19

Seroprevalence study to estimate percentage of Spanish population that has developed antibodies against COVID-19 to begin

Monday 27 April 2020

Through the regional health authorities and health centres, participants will be contacted to obtain samples. Those called to participate will begin on Monday, 27 April. "Participation is totally voluntary", stressed the Minister for Health, Salvador Illa, "but I would encourage all those contacted to take part in the study. The results will be tremendously useful for the whole of Spanish society".

By undertaking this study, the Ministry of Health and the Carlos III Health Institute, attached to the Ministry of Science and Innovation, in close collaboration with the regional governments, seeks to estimate the percentage of the Spanish population that has developed antibodies against the new SARS-CoV-2 (concept known as seroprevalence). The information obtained will be of tremendous importance in taking public health decisions throughout the country. The role of primary health centres will be particularly important during the process.

Development of study

In collaboration with the National Statistics Institute (Spanish acronym: INE), more than 36,000 Spanish households have been selected, so that the sample has participants from all age groups and all geographic locations. Accordingly, some 90,000 people will be invited to participate and it is estimated that the final sample will comprise a minimum of 60,000 people.

The design of the ENE-COVID study, which will be coordinated at a scientific level by the National Epidemiology Centre, considers the study of all those people who live in the same house, since this facilitates the representativity of the sample and, moreover, means that new infections spread by community transmission can be differentiated from those spread within the same household. This design is inspired by the recommendations of the World Health Organization (WHO) for studies into seroprevalence against SARS-CoV-2.

Furthermore, the ENE-COVID study seeks to dynamically evaluate the spread of the pandemic, to which end several visits will be made to participants to determine if they develop antibodies over the course of the term of the study.

How the study is performed

The sampling made provides a representative sample at a provincial, regional and national level. The size of the sample per province ranges from 900 people in the autonomous cities of Ceuta and Melilla to 6,000 people in Madrid, which allows the seroprevalence from COVID-19 to be estimated with sufficient accuracy in each province. In turn, the inter-provincial proportionality of the population will be maintained, which will lead to greater efficacy of the estimates at both a regional and a national level. The participants in the ENE-COVID study will respond to a brief questionnaire and will carry out blood tests to determine whether they have developed antibodies against the virus.

Firstly, they will be given a rapid test to determine antibodies in the blood through immunocromatography, which provides information to see if people are infected. This test will be carried out at home or in a health centres, depending on the spread of the epidemic and the circumstances of each family.

The sensitivity of this test, which only requires pricking a finger, is estimated at more than 80%, but this information on diagnostic precision has been obtained from very specific groups of patients and it is not known whether this can be extrapolated to the population as a whole. Hence, to ensure the reliability of the results and apply maximum methodological rigour, it has been considered highly recommendable to obtain a serum sample from all those patients that provide their consent. These samples will be obtained using the same technique as is routinely used for blood analyses (venepuncture, a pricking of the arm), which will be analysed using a more sophisticated and accurate serological technique than the rapid tests.

The analyses will be coordinated by the National Microbiology Centre and involve the collaboration of a large number of hospitals from around the country. Thanks to these two tests, which combine different levels of precision, an estimate of the population with the presence of antibodies to the virus will be obtained. Together with these two serological studies, the participants in the study will answer different questions on symptoms and potential sources of infection, by completing an epidemiological survey that will be performed through a web application to ensure the information is gathered quickly.

Information for participants

Households are selected at random. Each household will receive a phone call to inform the residents of the aims of the ENE-COVID study, to ask for consent and to arrange a house call or an appointment at a health centre. The study is voluntary, but it is very important for all those people selected to collaborate so that the information contained in the study is a true snapshot of the situation.

The information needed to see the existence of a preliminary diagnosis of COVID-19, the presence or history of symptoms compatible with this disease and the main risk factors known will be obtained from each participant. Special care is being taken over the information offered to participants, particularly to the interpretation of the test results. A document will be used with the informed consent of each member of the household, including one for children, so that they can understand what their participation consists of. In addition, a training programme has been set up, coordinated by the National Health School for healthcare workers that will perform the study.

Non official translation