Predicting the future is never easy. Add on the pressure of a global health crisis and it becomes an almost impossible task. But that’s the difficult task assigned to the numerous epidemiologists who are studying and attempting to model the continual spread of the novel coronavirus SARS-CoV-2.
In a release shared with the Science Media Centre (SMC), Francois Balloux, professor of computational systems biology and director of the University College London (UCL) Genetics Institute, summarized two key issues involved: “Predictions from any model are only as good as the data that parameterized it. There are two major unknowns at this stage. (1) We don't know to what extent COVID-19 transmission will be seasonal. (2) We don’t know if COVID-19 infection induces long-lasting immunity.”
Is COVID-19 seasonal?
In the same statement, Balloux ranked COVID-19 as the most serious public health threat since the 1918/19 influenza pandemic. The spread of that disease showed an initial decline after first emerging in March 1918. By October of the same year, it had returned with a vengeance with a second wave. A third peak in February 1919 also killed millions worldwide.
This seasonality of the influenza (combined with unique contributing factors in 1918, such as the return of soldiers after World War I) meant that the effects of that outbreak were felt over a very long time.
Martin Hibberd, a professor of emerging infectious disease at the London School of Hygiene and Tropical Medicine, also speaking to the SMC, pointed out that if a winter peak at the end of 2020 does emerge, countries’ prior policy responses will have huge ramifications: “My feeling is that the Government policy is taking a more forward planning approach than many and planning for the next nine months. This policy means that a peak of cases can be delayed and flattened, perhaps to last over the summer, when the NHS will be more prepared."
He continued, "With the worry that the disease could become more prevalent during the winter, as seasonal Influenza does, this strategy would give some herd immunity during these critical winter months. By pacing the social distancing, the hope is that they can be prolonged over these nine months. With more intense social distancing targeted at people who are likely to suffer more from the disease, this approach overall is hoped to save the maximum number of lives over this nine-months period. The worry with intense ‘lock-down’ approaches is that they may not be sustainable over this nine-month period."
Can COVID-19 patients be reinfected?
Unless, that is, people who have had COVID-19 are immune to reinfection. This is Balloux’s second point, and the uncertainty surrounding the answer is central to much of the anxiety facing public policymakers. Balloux points out that herd immunity to other coronaviruses, such as Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS), suggests the immunity may be short-lived, lasting only months.
If the novel coronavirus SARS-CoV-2 can rapidly change and re-infect the wider population, public policy in some countries would be left entirely wrong-footed. “Short-lived immunization would defeat both ‘flattening the curve’ and ‘herd immunity’ approaches. Devising an effective strategy would be even more challenging under low seasonal forcing. It would also considerably complicate effective vaccination campaigns,” says Balloux.