Host or pathogen-related factors in COVID-19 severity?

Abstract

Lucy Okell and colleagues1 observed that the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is currently in marked decline in many countries. Okell and colleagues suggest two possible explanations for this decline, namely the effect of lockdowns, physical distancing, and other interventions; or, alternatively, herd immunity. After analysing trends in cumulative deaths over time in many European countries that went into lockdown at different stages of their epidemic, and data obtained from serology studies on the proportion of the population that had the infection previously, Okell and colleagues found few data to support an explanation that relies on herd immunity. In the first explanation, there is a high risk of renewed transmission if interventions or behavioural modifications are relaxed. In the explanation regarding herd immunity, further declines in cases and deaths are to be expected even in the absence of interventions or behavioural modifications. We support Okell and colleagues for stating that identifying the most probable explanation is key to any future plans to lift physical distancing and travel restrictions. However, we would like to suggest a third explanation linked to changes in the pathogen, and hence in disease severity. SARS-CoV-2 has mutated.2, 3 This observation coincides with reports of a lower disease severity (measured in the number of days with symptoms and degree of pneumonia severity) in newer patients compared with those who contracted the disease earlier.4 If changes in population susceptibility or pathogen virulence are associated with a decline in disease severity, then less aggressive control interventions will be enough for future COVID-19 management. We encourage future epidemiological studies to include indicators of disease severity as predictor variables. Analyses that do not account for host-related or pathogen-related changes in disease severity should be interpreted with caution. We declare no competing interests.