The National ENE-COVID Seroprevalence Study concludes that 5% of the Spanish population have antibodies. This percentage has hardly changed during the three waves of the study: 5% during the first, and 5.2% during the second and third. The results confirm that, despite the significant impact of the pandemic in Spain, these figures are not high and reject any possible short-term herd immunity.
The release of these final results coincides with today’s publication of a scientific article in The Lancet, a magazine recognised as one of the most prestigious scientific publications in the world and with the second-strongest impact in the field of medicine.
Antibody trends is one of the issues examined by the two-month study and was one of the objectives. There was an overall seroconversion rate (individuals with no antibodies during the first wave who developed them during the course of the study after coming into contact with the virus) of 0.9% between round 1 and round 2 and a slightly lower percentage (0.7%) between round 2 and round 3. These figures reflect the low appearance of new infections following the lockdown.
Seronegativisation was also observed, i.e. the absence of IgG antibodies detectable in individuals who previously had them. Overall, this stood at 7.1% between round 1 and round 2, and approximately 14% when looking at the entire period of the study, although this last figure requires confirmation. The “loss” of antibodies was more frequent in individuals who had been asymptomatic (11% between round 1 and round 2, with more precise information) and much less frequent among participants with a positive PCR test (0.5%) and in those describing a sudden loss of taste or smell (2.6%).
The map of participants with COVID-19 symptoms (individuals with three or more symptoms or with a sudden loss of smell) in the last two weeks of the study provides an idea of more recent trends in the epidemic. While the percentage of symptomatic individuals fell substantially between the first and second rounds, a slight increase was seen in the last round. This could be the result of increased mobility in the population following the lockdown period. It is possible that a percentage of these patients could be COVID-19 cases in early stages of the disease. Continued monitoring of participants will enable these questions to be answered.
The authors of the article in The Lancet believe that the results obtained in Spain, a country with an intense epidemic wave in which seroprevalence figures are low, reflect the difficulty in achieving herd immunity in the short term. In this regard, the comments that accompany the article in The Lancet by researchers at benchmark centres in Geneva (Dr. Isabella Eckerle & Dr. Benjamin Meyer) indicate that “it would be rather unethical to subject the population and the healthcare system to increased pressure in order to achieve this herd immunity”.
In short, these results combined with the presence of a substantial percentage of asymptomatic infections underline the need to maintain the public health recommendations on social distancing, the use of face masks and hand washing.
As stated in the publication and in the communication of preliminary data from the first waves, there is significant geographic variations in seroprevalence percentages. Some provinces stand at 2-3% while others have over 10%, with the highest figures being recorded in the central regions of mainland Spain. This percentage is also higher in population centres with more than 100,000 inhabitants when compared with municipalities with a lower population (6% and 4%, respectively).
No differences were observed between men and women, while, in terms of age, the prevalence of IgG antibodies against SARS-CoV-2 is slightly lower in children and teenagers (around 3.5%), with insignificant variations in adults. The article published in The Lancet also highlights the higher prevalence of antibodies in healthcare professionals (10%) and in care home and socio-healthcare centre employees (7.7%).
The final results of the study confirm that between 74% and 89% of participants claiming to have had a positive PCR test at least two weeks before the study have IgG antibodies against the coronavirus. Among those individuals who claim to have shown symptoms compatible with the disease, seroprevalence increased with the number of symptoms and is particularly high in those individuals claiming to have experienced a sudden loss of taste and/or smell (40%-41%). Finally, it is confirmed that approximately 2.5% to 2.8% of participants claiming to have shown no symptoms did present IgG antibodies, underlining the existence of asymptomatic infections.
The participants who claimed to have come into contact with a confirmed COVID-19 case or someone with compatible symptoms shows a higher prevalence of antibodies than the general population. It is worth noting the prevalence in individuals who have lived with a confirmed COVID-19 patient, which stands between 27% and 31% in the various rounds. Those living with individuals showing symptoms compatible with the disease also presented a higher than average seroprevalence (15%). Seroprevalence in individuals claiming to have come into contact with a confirmed case outside of the home ranges between 10% and 15% in the various rounds.
The three rounds of this study, which was promoted by the Ministry of Health and the Carlos III Health Institute (the latter managed under the Ministry of Science and Innovation) and in which the healthcare systems of all the autonomous regions and the autonomous cities of Ceuta and Melilla took part, were conducted from 27 April to 11 May; from 18 May to 1 June; and from 8 June to 22 June.
It included a total of 68,296 participants, of which 54,858 took part in all three rounds, representing an almost 90% level of adherence. Furthermore, 91% of participants provided at least one blood sample for analysis.
In total, 186,908 rapid tests were performed (plus 9,755 in the island-specific study) and 165,176 blood samples were collected (plus an extra 9,130 in the island-specific study).