[danielharan]

Many of those people testing positive were presymptomatic (some even developed symptoms during the follow-up period). It typically takes several days to develop symptoms.

Also worth considering is the period mentioned in the correspondence is one in which there was very fast exponential growth. When more of the positive cases are recent, it's only normal that they don't show symptoms yet.

Besides the test used showing a lot of false positives, the population of pregnant women looked at had a much higher rate of symptomatic infections 1 month ago than the entire population does today... we should be very cautious about concluding much from that sample.

https://www.nejm.org/doi/full/10.1056/NEJMc2009316