By Prof. Ben Cowling, School of Public Health, The University of Hong Kong
Hong Kong has faced a surge in COVID-19 transmission in the past month. Fortunately, we have seen daily case numbers decline in the past few days, and it looks like we have now passed the peak in our “third wave” locally. We have been monitoring the real-time reproductive number on our HKU COVID-19 dashboard, and we estimate that the daily reproductive number dropped below 1 on 22 July, around a week ago. However, the local reproductive number has not dipped very far below 1.
A reproductive number of exactly 1 would mean on average one case is infecting one further person, and daily numbers of cases would stay the same from week to week. The lower the reproductive number is below 1, the better. As of a week ago, the reproductive number was in the range 0.8 to 0.9, much higher than it was after successful control measures ended our community epidemic in March.
There are three strategies working in combination to suppress COVID-19 transmission in Hong Kong. Strategy #1 is personal behaviours including face mask use, attention to hand hygiene, and other behaviors to avoid risk such as avoiding crowded places and staying at home more often. We have been doing really well at this for the past 6 months. Strategy #2 is “test+trace”. We do a lot of COVID testing in Hong Kong, we isolate any cases that are identified, we aim to trace their close contacts who might have been exposed, and we aim to put those close contacts in quarantine. The idea of contact tracing and quarantine is to get ahead of virus transmission. Strategy #3 is social distancing or physical distancing. In Hong Kong, complete lockdowns of society are probably not feasible, but public and private sector employees have been encouraged to work at home; large gatherings of people have been discouraged and bars, nightclubs, gyms and leisure facilities have been closed. Schools have also been closed for most of the past 6 months, with children taking their classes online.
Back in March, strategies #1 plus #2 reduced transmission, but were not enough to stop an epidemic. At the end of June, #1 and #2 were still in good use locally, but were also not enough to stop an epidemic. To be completely clear, transmission would have been higher if we had not all been wearing masks. Transmission would have been higher if we had not been doing test and trace. But these two strategies were not enough to stop an epidemic in March, and again not enough to stop an epidemic in July.
Soon after social distancing measures were enacted by the government, which perhaps motivated further behavioural changes by the local population (improvement in strategy #1), the reproductive number finally dropped below 1.
The first measures were closure of bars and leisure facilities from 13 July, when we had 40-50 cases per day. A week later, many civil servants were asked to work at home, when we had 100 cases per day. Some private businesses followed the government’s lead and also asked their staff to work at home.
Why hasn't the reproductive number come down even lower, why is it stuck at 0.8 to 0.9 ? I agree with the concern posed by Dr Leung Chi-chiu that contact tracing and quarantine might not currently be operating at 100%. There are many “unlinked” cases reported every day, whereas at this stage in an epidemic we would expect that - in theory - most cases could be linked with previous cases, it is just a (time consuming) matter of identifying the links. Dr CC Leung asked whether perhaps the contact tracing team has been overwhelmed by 100 plus cases per day for almost 2 weeks. If this issue could be resolved, perhaps by adding more manpower to the contact tracing team, we should see the reproductive number decline further in the coming weeks.
Social distancing inconveniences 7 million people, while contact tracing only inconveniences the small number of close contacts identified for each case. If test+trace can get back up to speed, we should be able to relax the social distancing measures sooner.
Looking forward, we recognise that after local cases come down to zero, we will be able to relax social distancing measures (hopefully sometime in September). At some point later in the year infections will get back into the community, triggering another epidemic wave.
If or when that were to happen, I think it would be important to implement social distancing measures relatively quickly, to protect our “test+trace” capacity from being overwhelmed again.