19/03/2020

To mitigate or suppress the spread of the coronavirus?

Reproduction from the IMPA Science & Mathematics blog, from O Globo, coordinated by Claudio Landim.

Claudio Landim, Deputy Director of IMPA

Has the federal government finally become convinced of the importance of theoretical research and the need to consult the scientific community in order to develop public policies? The current epidemic illustrates this role well.

The English government initially proposed combating the spread of the coronavirus through herd immunity, based on the fact that a virus cannot proliferate if 60% or more of the population is immune. [1] A mathematical model of epidemic spread [2], developed by a group at Imperial College London, convinced them to change strategy.

The article compares the effects of two approaches to containing the epidemic: mitigation and suppression.

The fundamental parameter in studying the spread of an epidemic is its average reproduction rate. It represents the average number of individuals infected by each infected person. If there are three infected individuals and the first transmits the virus, during their contagious period, to two other people, the second to three, and the third to none, the average reproduction rate is equal to (2+3+0)/3= 1.66.

Naturally, this rate varies over time and depends on several factors, such as the environment, demographics, the genetic makeup of the population, etc. Measures such as social isolation, confinement, or population immunization can decrease it. This rate allows us to estimate the future number of infected people. As long as it is greater than 1, the number of infected people will increase exponentially. Conversely, if it is less than 1, the epidemic will disappear quickly.

A suppression policy consists of adopting measures to reduce the average reproduction rate to a value less than 1, while a mitigation policy consists of reducing it to values close to, but greater than, 1.

China and South Korea adopted suppression policies, while the English government proposed, before article [2], mitigation policies.

The study was developed taking into account the demographic, social, and economic conditions of the US and England. It is based on estimates of the transmission rate, which may change.

If nothing were done, the peak of the epidemic would occur three months after its start. By the end, 80% of the population would have been infected. There would have been 510,000 deaths in England and 2.2 million in the United States.

A mitigation policy, combining home isolation of suspected cases, home quarantine of those cohabiting with suspected cases, and social distancing of the elderly and other at-risk groups, should reduce the peak demand for medical care by two-thirds and deaths by half. The resulting mitigated epidemic would result in hundreds of thousands of deaths and repeated overburdening of the healthcare system, particularly intensive care units.

A suppression policy would require, in addition to the measures described above, the closure of schools and universities until a vaccine becomes available. Therefore, for a minimum period of 18 months. Intermittent social isolation measures could be taken during this period, relaxing confinement guidelines and reopening schools and universities at certain intervals.

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