Has China faced only a herald wave of SARS-CoV-2?

The attack rate of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) calculated by mathematical models, from estimates of the basic reproduction number, R0, of 2–3, suggests that 50–60% of the population should eventually be infected because the population seems to be entirely naive to the new virus. The observed attack rate on board the Diamond Princess cruise ship remained slightly below 20% (705 of 3711 passengers and crew members became infected). It is of upmost importance to know whether the SARS-CoV-2 outbreak in China is subsiding, as local authorities and the entire international community might wish. With 80 026 COVID-19 cases officially reported from China as of March 2, 2020, the proportion of the population affected remains far from 50%, or even 20%, of China's 1·4 billion people. Has China just experienced a herald wave, to use terminology borrowed from those who study tsunamis, and is the big wave still to come?

Serosurveys can help answer these questions precisely. To serosurvey the outbreak would involve testing sera of blood samples from the most representative sample of the population at the epicentre of the epidemic, Wuhan. Serology analysis with neutralising antibodies from the 1000 people could allow for the rate of SARS-CoV-2 infections to be estimated with good accuracy. This rate could be extrapolated to the city's entire population and thus inform more precisely whether the provisional attack rate during this period was a few cases per thousand or perhaps affected 1–2% of the population, 20%, or more. Serosurveys should be seen as polls before elections; they can be repeated several times, week after week, to monitor the epidemic precisely.

There is no reason to wait for the end of the epidemic before doing serosurveys. The results would be tremendously informative to China, first and foremost, and to the entire international community, on the risk of big secondary epidemic waves.