HSPH Researchers Develop Model to Evaluate Spread of Covid-19 Variants

Researchers at the Harvard T.H. Chan School of Public Health developed a mathematical model to explore the population-level impacts of various SARS-CoV-2 variants and the effects of vaccination in combating them.

The study’s model, described in a paper last month in the journal Cell, worked to understand how different strains of the coronavirus interact at the population level with vaccination and other non-pharmaceutical prevention methods, such as wearing masks.

The team found that variants with some immune system evasion — termed partial immune escape — most often results in a less severe impact or fails to spread widely at all, while enhanced transmissibility in a variant increases infections and limits the time available for vaccination to impact case numbers.

However, the effect of variants is most significant when both of these changes occur simultaneously. The recent Delta variant likely falls into this category, which may help explain its significant impact on the spread of the virus, per the study.

HSPH postdoctoral research fellow Mary E. Bushman, who led the study, told the Harvard Gazette, a University-run publication, that the researchers are still unsure how Omicron — the newly dominant strain in South Africa — fits into the study’s framework.

“At the moment, we don’t have the necessary information to distinguish between increased transmissibility, immune escape, and other factors,” Bushman said in a Nov. 29 interview with the Gazette.

Bushman and others are continuing to run simulations to determine the potential impacts of the Omicron variant.

While the different strains of SARS-CoV-2 may impact vaccine efficacy for an individual, that is not necessarily the case at the population level. Despite rising case numbers, the number of infections avoided due to vaccination remains consistent within the population as a whole.

Bushman explained in an email that vaccines are still able to prevent a great number of infections, even if variants with partial immune escape are present.

“That’s partly because we were looking at the total number of infections prevented, including non-variant infections, but it’s also because when a lot of people receive a partially effective vaccine, you still prevent a lot of infections,” Bushman wrote.

A leading hypothesis suggests that variants are the result of chronic SARS-CoV-2 infections that fail to clear the host’s body — a byproduct of increased spread of the virus, according to Bushman.

Bushman wrote the researchers believe vaccination is a top priority to limit chronic Covid-19 infections and the development of future variants.

“Any infection that you prevent by vaccination might be the predecessor to a future variant of concern. In my opinion, though, that’s a secondary justification for equitable access to vaccines. The primary justification is that it’s a moral imperative,” Bushman wrote.