The virulence of a virus is the degree of harm it causes to its host. Virulence can be influenced by many factors, such as the virus's ability to evade the immune system, to replicate efficiently, to cause tissue damage, to induce inflammation, and to transmit to new hosts¹. Virulence can also change over time as the virus evolves in response to natural selection and other evolutionary forces².

There is no simple answer to why COVID-19 became less virulent, but some possible explanations are:

- The virus adapted to its human host and became more efficient at infecting cells without causing excessive damage. This could increase the virus's transmission and survival, as well as reduce the host's immune response and inflammation¹².
- The virus accumulated mutations that reduced its fitness or impaired its function. This could happen by chance or by error-prone replication, especially under selective pressure from the immune system or antiviral drugs¹².
- The virus faced competition from other variants that were more transmissible or more resistant to immunity. This could reduce the virus's prevalence and exposure, as well as increase the genetic diversity and recombination of the viral population¹³.
- The host population developed immunity to the virus through vaccination or previous infection. This could limit the virus's replication and transmission, as well as select for less virulent variants that can coexist with the host without triggering severe disease¹⁴.

These explanations are not mutually exclusive, and they may interact with each other in complex ways. The virulence of COVID-19 may also vary depending on the host's age, health status, genetic background, and environmental factors¹. Therefore, it is difficult to predict how COVID-19 will evolve in the future, and whether it will become more or less virulent again.

Source: Conversation 6/27/2023
(1) SARS-CoV-2-specific virulence factors in COVID-19 - PubMed. https://pubmed.ncbi.nlm.nih.gov/33085084/.
(2) SARS-CoV-2 variant biology: immune escape, transmission and fitness. https://www.nature.com/articles/s41579-022-00841-7.
(3) Will the Coronavirus Evolve to Be Less Deadly?. https://www.smithsonianmag.com/science-nature/will-coronavirus-evolve-be-less-deadly-180976288/.
(4) Comparing infectivity and virulence of emerging SARS-CoV-2 variants in .... https://www.thelancet.com/journals/ebiom/article/PIIS2352-3964%2821%2900196-1/fulltext.