Why Covid-19 isn’t over and how you can help
Last week, a fantastic team of colleagues and I at the Harvard T. H. Chan School of Public Health published a paper in which we show that one-time social distancing measures are unlikely to contain Covid-19, the disease caused by the novel coronavirus SARS-CoV-2, without other interventions like a treatment or vaccine. In this post, I hope to explain some of the intuition behind our findings and emphasize that stopping the spread of this coronavirus will be a marathon, not a sprint.
Three key components drive how quickly a disease spreads:
- How contagious is the disease?
- How often are people contacting one another in a way that could spread the infection?
- What proportion of the population is susceptible? Here, susceptibility simply means that one could get the infection if exposed to SARS-CoV-2; the opposite of susceptibility is immunity. Immunity may be gained by experiencing the infection or receiving an effective vaccine. Although having Covid-19 likely confers some immunity to the virus, the strength and duration of immune protection are still open questions.
The combination of these three factors is called the effective reproduction number (sometimes called Reff or Rt). This is the average number of people that an infectious person will spread the virus to. We need this number to be less than one for epidemic growth to slow.
To prevent overwhelming healthcare systems, unprecedented social distancing measures have been adopted to push down (2) as much as possible. In many places, it looks like these efforts have been successful in reducing Rt. However, these measures have no impact on (1). Also, while data is currently limited, it is likely that only a small proportion of the population has gained immunity, so (3) has not changed much either. Thus, as we lift restrictions and increase (2), we can also expect the spread of the virus to increase. If we were to snap back to pre-lockdown levels of (2) tomorrow, the virus would spread nearly as quickly as it did prior to social distancing measures, causing a second peak of infections if no other measures were in place.
To be clear, I am in no way advocating that we simply decrease (3) through uncontrolled spread of the virus in order to “get it over with”. This would lead to devastating loss of life, catastrophic burden on healthcare systems, and undoubtedly negative social and economic fallout. Instead, gradually lifting measures with widespread testing to detect new cases will allow us to determine the effectiveness of specific interventions. If we do see a rise in cases, we may need to restart certain measures to slow spread. It may take multiple rounds of starting and stopping measures in this way to develop local approaches that will allow us to safely build population immunity while resuming as many normal activities as possible. With these strategies in place, it could take 12–18 months to build up enough immunity in the population to finally stop the spread of the virus.
Testing remains limited in many places, but crowd-sourced surveillance platforms can provide an early signal of increasing numbers of people with Covid-like symptoms. As communities around the world begin to ease social distancing measures, your contributions to HowWeFeel are crucially important to help scientists and policymakers rapidly detect disease resurgences. Remember that it is important to check-in both when you feel well and when you experience symptoms to help researchers accurately understand disease trends.
Many thanks to Stephen Kissler for his helpful comments on this piece. The illustration shown here has been adapted from Figure 4 of our paper and the work of Lindsay Diamond (@neurdy).
