Watching the daily increase in Covid-19 cases in HK is scary. However, a more useful analysis is the ‘epidemic curve’, which looks at the date of symptom onset of cases rather than the date of case detection.

Unlike date of case detection, date of symptom onset gives actual epidemiological information about the circulation of virus in the community. Therefore, epidemic curves provide a better view of a community outbreaks than daily case numbers.

Here, I have plotted the epidemic curve of 60 Covid-19 cases in HK with available information (updated as of 22/2/20). I have extracted the symptom onset dates from CHP press releases.

The following messages emerge:

1. Intense local Covid-19 transmission in HK was already occurring around the Chinese New Year holiday.
2. The outbreak in HK is a propagated epidemic with successive waves of infection occurring at intervals of 3-6 days, which is close to the mean incubation period of the virus (Peaks: 28/1 – 30/1 –> 1/2-4/2 –> 7/2-8/2 –> 10/2-12/2)
3. Encouragingly, the size of the peaks in the last week have been smaller than in the preceding two weeks, despite HK still having a large susceptible population. However, this could be due to long lag-time between symptom onset and diagnosis.

The next week or so will define the direction of this epidemic.

Heartfelt thanks to colleagues at the Centre for Health Protection for their painstaking field work

Stay hopeful. Stay masked. Stay safe.