Mapping the Future of the COVID-19 Pandemic
A drop-off in virulence is possible for the SARS-CoV-2 virus -- once enough of the global population has been vaccinated or exposed to it, according to a new study from scientists at Emory University and The Pennsylvania State University.
The findings, published Tuesday in Science, use immunological and epidemiological data on endemic, cold-causing human coronaviruses to model transmission in a world where most adults have had an initial immune exposure to the virus, via infection or vaccine.
The authors predict that SARS-CoV-2 – unlike more deadly coronavirus cousins SARS and MERS – may eventually spread like the most common, non-deadly human coronaviruses do, providing initial exposure to children under the age of 5, when immune responses are less likely to result in serious illness.
According to the model, a virus’s infection fatality ratio is mostly dependent on how severe childhood infections are, and a coronavirus that with the morality of MERS infections but with the endemic pattern of COVID-19 could only by curbed with mass childhood vaccinations.
The model is based on four existing endemic human coronaviruses. Previous studies have shown that reinfection throughout adulthood after a childhood infection is commonplace, and may even continuously improve human immune response to the strains. As a result, the authors warn that the apparent low rates of reinfection with COVID-19 recently observed could actually dampen this drift toward lower virulence.
Although the paper does not offer a timeline for the transition from COVID-19 pandemic to a safer endemic, it does suggest that mass vaccinations could be halted at the end of this transient period, depending on the immunological response to vaccines.
If the response to vaccines mimics the response to common cold-type human coronavirus infections – specifically, primary introduction to the virus in early childhood and short periods of protection against reinfection, followed by long-lived protection against severe disease – COVID-19 could more quickly become endemic, according to the study.
Other experts have cautioned that this model is only one possible scenario. Jennifer Gommerman, a professor of immunology at the University of Toronto, told the New York Times that introducing a vaccine via intravenous injection meant the body is not introduced to natural pathogens in the upper respiratory tract like usual, so it is remains unclear whether the immune reaction would be consistent enough for the model to hold. And the lack of data for endemic human coronaviruses in naïve older people means comparisons might be premature, said Marc Lipsitch, a professor of epidemiology at Harvard University’s Harvard T.H. Chan School of Public Health, to the Times.