The COVID-19 pandemic: key considerations for the epidemic and its control

Abstract

The response to the ongoing COVID-19 pandemic has been characterized by draconian measures and far too many important unknowns, such as the true mortality risk, the role of children as transmitters and the development and duration of immunity in the population. More than a year into the pandemic much has been learned and insights into this novel type of pandemic and options for control are shaping up. Using a historical lens, we review what we know and still do not know about the ongoing COVID-19 pandemic. A pandemic caused by a member of the coronavirus family is a new situation following more than a century of influenza A pandemics. However, recent pandemic threats such as outbreaks of the related and novel deadly coronavirus SARS in 2003 and of MERS since 2012 had put coronaviruses on WHOs blueprint list of priority diseases. Like pandemic influenza, SARS-CoV-2 is highly transmissible (R₀ ~ 2.5). Furthermore, it can fly under the radar due to a broad clinical spectrum where asymptomatic and pre-symptomatic infected persons also transmit the virus-including children. COVID-19 is far more deadly than seasonal influenza; initial data from China suggested a case fatality rate of 2.3%-which would have been on par with the deadly 1918 Spanish influenza. But, while the Spanish influenza killed young, otherwise healthy adults, it is the elderly who are at extreme risk of dying of COVID-19. We review available seroepidemiological evidence of infection rates and compute infection fatality rates (IFR) for Denmark (0.5%), Spain (0.85%), and Iceland (0.3%). We also deduce that population age structure is key. SARS-CoV-2 is characterized by superspreading, so that ~10% of infected individuals yield 80% of new infections. This phenomenon turns out to be an Achilles heel of the virus that may explain our ability to effectively mitigate outbreaks so far. How will this pandemic come to an end? Herd immunity has not been achieved in Europe due to intense mitigation by non-pharmaceutical interventions; for example, only ~8% of Danes were infected across the 1^st and 2^nd wave. Luckily, we now have several safe and effective vaccines. Global vaccine control of the pandemic depends in great measure on our ability to keep up with current and future immune escape variants of the virus. We should thus be prepared for a race between vaccine updates and mutations of the virus. A permanent reopening of society highly depends on winning that race.

Keywords: COVID-19; Coronavirus; mortality; pandemic; superspreading.

Fig. 1

Timeline of respiratory viral pandemics in the 20th and 21st century. After a century of influenza A pandemics, a pandemic coronavirus emerged. In 1918, 1957, and 1968 pandemics are thought to have arisen from birds in Asia, whereas in 2009 originated in Mexican pigs. The origin of SARS‐CoV‐2 is thought to be Chinese bats. The colored labels indicate the pathogen responsible for the disease in question.

Fig. 2

Simulations of an agent‐based model with network and superspreaders (see full model and assumptions in [47]). (A) A single infection tree—the result of a model simulation of superspreading. The epidemic spreads due to a small proportion of individuals who are highly infectious, while the majority do not transmit the disease. (B) Effect of mitigating in the public domain to reduce opportunities for superspreading. If a sizable proportion of infections are caused by superspreaders, the simulations show that just reducing contacts in the public space (that is, outside households and workplaces/schools), has a large mitigation effect (right subpanel); but without superspreaders in the model, not much is gained (left subpanel). Data for panel B from [47]. In these simulations, superspreaders are individuals with a higher personal reproductive number, thus having the potential to transmit the disease to many in an unmitigated scenario. Drastically reducing the number of different persons that one meets (by e.g., banning large gatherings) thus has an outsized effect in a disease characterized by superspreading, providing an opportunity for improved mitigation. The theoretical background for this effect is explored in ref. [49].

Fig. 3

A sustainable control strategy in Sweden? On March 28th, Sweden introduced a ban on events >50 persons and the daily numbers of deaths started to decline a few weeks after [96]. On October 8, some gatherings were again allowed up to 300. Many other factors were in effect in Sweden, including working from home, less traveling, more effective shielding of the elderly, closed universities, and the seasonal changes in temperature and humidity. But borders remained open, as did schools for children up to 16 years of age in this time period.