Response: Seroprevalence of SARS-CoV-2 antibodies among first-trimester pregnant women during the second wave of the pandemic in India.

We read with interest the Letter to the Editor from Sookaromdee & Wiwanitkit regarding our recent publication on seroprevalence of SARS‐CoV‐2 antibodies among first‐trimester pregnant women during the second wave of the pandemic in India.

The comment regarding false positive antibody tests is valid. However, antibody testing using the new generation kits is highly specific, excepting reported cross‐reactions with other coronaviruses. The virologic surveys in India during the study period do not show a prevalence of non‐COVID‐19 strains in India during this time (unpublished data). The reference quoted in the Letter to the Editor for false positives in dengue discusses a lateral flow technique. We, on the other hand, performed antibody testing using the Vidas platform, which gives a quantitative reading. It detects the presence of antibodies using the S1/receptor‐binding domain (RBD) of the SARS‐CoV‐2 S protein, with high sensitivity (PPA*) and specificity (NPA**).

Tables 1 and 2 show the US FDA findings for sensitivity and specificity for the Biomeriux Vidas IgG and IgM platforms.

TABLE 1

US FDA findings for sensitivity and specificity for the Biomeriux Vidas SARS‐CoV‐2 IgG platform

Antibody	Performance measure	Estimate of performance	95% confidence interval
IgG	Sensitivity (PPA)	100% (29/29)	(88.3%; 100%)
IgG	Specificity (NPA)	99.9% (988/989)	(99.4%; 100%)

TABLE 2

US FDA findings for sensitivity and specificity for the Biomeriux Vidas SARS‐CoV‐2 IgM platform

Antibody	Performance measure	Estimate of performance	95% confidence interval
IgM	Sensitivity (PPA)	100% (23/23)	(85.7%; 100%)
IgM	Specificity (NPA)	99.4% (306/308)	(97.7%; 99.8%)

In a study conducted to evaluate Vidas‐based antibody testing for COVID‐19, (CE‐marked, authorized, automated, qualitative assays for the detection of SARS‐CoV‐2‐specific IgM and IgG), both assays showed high within‐run and within‐laboratory precision (coefficients of variation < 11.0%) and very low cross‐reactivity toward sera of patients with a past common coronavirus or respiratory virus infection. Clinical specificity determined on up to 989 pre‐pandemic healthy donors was ≥99% with a narrow 95% confidence interval for both IgM and IgG assays. Clinical sensitivity was determined on up to 232 samples from 130 reverse transcriptase PCR (RT‐PCR)‐confirmed SARS‐CoV‐2 patients. The positive percent agreement (PPA) with SARS‐CoV‐2 PCR reached 100% at ≥16 days (Vidas SARS‐CoV‐2 IgM) and ≥32 days (Vidas SARS‐CoV‐2 IgG) of symptom onset. Although anti‐SARS‐CoV antibodies were reported to bind cross‐reactively to the S, S1, RBD, and N proteins of SARS‐CoV‐2, this cross‐reaction is of less significance because there has been no SARS case report since 2004, and the number of infections with SARS‐CoV was limited to 8096 worldwide according to WHO.