Seroprevalence of SARS-CoV-2 antibodies among pregnant women in Estonia: A call for epidemiological studies.

Sir,

On 7 April 2020, Mehreen Zaigham and Ola Andersson published a systematic review of maternal and perinatal outcomes in 108 pregnancies with Covid‐19 concluding that careful monitoring of such pregnancies is warranted. We would like to emphasize the need to assess objectively the impact of the novel Severe Acute Respiratory Coronavirus Type 2 (SARS‐CoV‐2) causing Covid‐19 disease on pregnancy and perinatal outcomes by conducting epidemiological studies among pregnant women.

Almost all persons infected with SARS‐CoV‐2 test positive for antiviral immunoglobulin type G (IgG) antibodies 10‐20 days after being infected. The proportion of seropositive persons among pregnant women has been used as a proxy of the disease prevalence among the general population. , In Spain, the population‐based seroprevalence of SARS‐CoV‐2 for the period of 27 April to 11 May was 4.6%.

During the Covid‐19 pandemic, the routine first trimester combined screening (OSCAR test) was temporarily replaced by non‐invasive prenatal screening test (NIPT) for all pregnant women in Estonia from 13 March to 18 May 2020. Blood samples used for NIPT were collected in The Competence Center on Health Technologies and the serum residuals of 433 women were used for detecting SARS‐CoV‐2 IgG antibodies in SYNLAB Estonia using the chemiluminescent microparticle immunoassay (CMIA) technique on ARCHITECT i2000SR. The IgG antibody level above 1.4 index (S/C) was defined as a positive result. The study was approved by the Research Ethics Committee of the University of Tartu (decision No. 315/T‐20, 18 May 2020). The study was funded by the Estonian Research Council (grants Nos. IUT34‐16 and IUT34‐17).

For the current analysis, the residual blood samples from NIPT testing were collected during the period from 4 May to 10 June 2020. Participants represented all regions in Estonia. The mean age of study participants was 31 years (SD 5.89), mean duration of pregnancy at the time of testing was 11 gestational weeks (SD 2 weeks) and mean body mass index (BMI) of women was 24.53 (SD 0.31). Only two women (0.46%) tested positive for SARS‐CoV‐2 IgG antibodies.

The cumulative incidence of SARS‐CoV‐2 RNA‐positive cases from nasopharyngeal swabs by 17 April 2020 among the general population aged 30‐34 years in Estonia was 9.9 per 10 000 (the cumulative number of all positive tests by 17 April 2020 was 1510 (3.8%) of 39 583 tests with 60% of tests taken from females and 55% of positive tests among females). The seroprevalence among pregnant women of the same age about a month later in our study was nearly five times higher. This suggests that serologic methods can be more informative of the disease burden than case‐based viral nucleic acid testing from nasopharyngeal swabs using PCR.

In comparison with available data from Spain, seroprevalence of SARS‐CoV‐2 antibodies among pregnant women in Estonia was 10 times lower than among the general population in Spain, indicating the possibility of regional differences in the incidence of COVID‐19 across Europe. In Estonia, this epidemic is well contained so far, with no excess mortality due to COVID‐19.

In conclusion, serologic studies among pregnant women could not only be useful in following the trajectory of the COVID‐19 epidemic within a country but could also provide a more standardized way to compare the burden of infection among different countries. Only well‐planned and well‐conducted epidemiologic studies among pregnant women will make it possible to determine the real effect of the SARS‐CoV‐2 virus on perinatal outcomes.