Reviewer #1 (Public Review):
The paper "Insights from a Pan India Sero-Epidemiological survey (Phenome-India cohort) for SARS-CoV2" reports a longitudinal survey of about 10000 subjects from laboratories of the CSIR (India) who consented to be tested for antibodies to SARS-CoV-2, across August and September 2020. The methodology is a standard one, using the Roche kit to test for antibodies to the nucleocapsid antigen with a followup to detect neutralising antibodies using the GENScript Kit. A questionnaire for all participants asked about age, gender, pre-existing conditions and blood group, among other questions.
The principal results of the study were:
1) An overall seropositivity of 10.14% [95% CI: 9.6 - 10.7] but a large variation across locations
2) Virtually all of the seropositive exhibited neutralising activity
3) Seropositivity correlated with population density in different locations
4) A weak correlation was seen to changes in the test positivity across locations
5) A large asymptomatic fraction (~75%) who did not recall symptoms
6) Of those symptomatic, most reported mild flu-like symptoms with fever
7) A correlation with blood group, with seropositivity highest for AB, follow by B, O and A
8) A vegetarian diet correlated with reduced seropositivity
9) Antibody levels remained constant for 3 months across a sub-sample white neutralising activity was lost in ~30% of this subsample. Over a longer period, in a still smaller subsample of those tested at 3 months, anti-nucleocapsid antibody levels declines while neutralising antibody levels remained roughly constant
10) There is a reasonable agreement with the results of the second Indian serosurvey which obtained a seroprevalence of about 7% India-wise, although excluding urban hotspots.
The deficiencies of this study are:
1) This is a very specific cohort, largely urban, with - presumably - relatively higher levels of education. It is hard to see how this might translate into a general statement about the population
2) The presentation of Figure 1 was quite confusing, especially the colour coding
3) It is surprising that the state of Maharashtra shows only intermediate to low levels of seropositivity, given that the impact of the pandemic was largest there and especially in the city of Pune. There have been alternative serosurveys for Pune which found much higher levels of seropositivity from about the same period.
4) The statement "Seropositivity of 10% or more was associated with reductions in TPR which may mean declining transmission": For a disease with R of about 2, this would actually be somewhat early in the epidemic, so you wouldn't expect to see this in an indicator such as TPR. TPR is also strongly correlated with amounts of testing which isn't accounted for.
5) The correlation with vegetarianism is unusual - you might have argued that this could potentially protect against disease but that it might protect against infection is hard to credit. Much of South Asia is not particularly vegetarian but has seen significantly less impact
6) On the same point above, it is possible that social stratification associated with diet - direct employees being more likely to be vegetarian than contract workers - might be a confounder here, since outsourced staff seem to be at higher risk.
7) There may be correlations to places of residence that again act as confounders. If direct employees are provided official accommodation, they may simply have had less exposure, being more protected.
8) The correlations with blood group don't seem to match what is known from elsewhere
9) The statement that "declining cases may reflect persisting humeral immunity among sub-communities with higher exposure" is unsupported. What sub-communities?