Strengths and limitations
The prospective design is a strength of our study. Participants were
compared with all non-participants by a retrospective registry linkage,
enabling us to examine whether participants were representative of all
pregnant women with COVID-19 at the index hospital during the study
period. Additionally, tests for SARS-CoV-2 in Denmark are free of charge
and performed at public test centres reporting the results to national
databases, which reduced the number of undetected cases of COVID-19.
Finally, we used a serological assay with a high diagnostic accuracy
with a sensitivity of 95.3% and a specificity of 100% (13).
The study has several limitations. We included 77.8% of eligible women.
However, participants and non-participants did not differ with regards
to background characteristics. We had no samples from mothers or cord
blood between eight and 16 days after a positive maternal pharyngeal
swab, entailing the exact timing of seroconversion among pregnant women
with COVID-19 unknown. Further, most vaginal swabs were kept at
-20◦C until analysis, possibly causing a deterioration
of SARS-CoV-2 during storage, decreasing our ability to detect
SARS-CoV-2 in the vaginal samples. However, a previous study found that
SARS-CoV-2 RNA was stable when stored at -20°C for up to 84 days (14).
Lack of infection status from the participants’ partners is a
limitation, since it is not fully evaluated whether SARS-CoV-2 can be
transmitted through semen and cause detection of SARS-CoV-2 in the
vagina (15). We measured total antibodies and did not discriminate
between IgG and IgM. Finally, three participants were not tested by
pharyngeal RT-PCR (one by an antigen-test and two at private test
centres), thus exact details about analysis methods are lacking.
However, the participants were positive for SARS-CoV-2 antibodies.