Individuals infected with SARS-CoV-2 after three doses of vaccine COVID-19 are much more likely to have the Omicron variant than the Delta variant, due to Omicron's ability to bypass the immune system, according to US data.
According to a related Nature article, one way in which viral variants can become dominant is to increase their transmissibility through faster reproduction or enhanced ability to spread outside the body. Another path to dominance is to use an increased ability to escape immune systems that have learned - through infection or vaccination - to recognize the virus.
Delta achieved dominance with the previous method: the virus could reproduce much faster and at higher levels than previous variants. "Delta did a lot of things through brute force," said Nathan Grubaugh, an epidemiologist at the Yale School of Public Health in New Haven, Connecticut. Omicron appears to reproduce even faster than the Delta, but the number of virus particles in the body never reaches the same level, suggesting that the advantage of the variant lies elsewhere.
Delta vs Omicron
Grubaugh and colleagues studied the results of 37.877 polymerase chain reaction tests in samples collected in Connecticut for two weeks in mid-December 2021, when Delta and Omicron accounted for about 50% of the cases. COVID-19 in the area. The team determined which variant had detected each test and correlated this data with the vaccination status of the infected person.
In a pre-release published Jan. 25, the researchers reported that infected individuals who were either not vaccinated or had only received one dose of vaccine were more likely to have Delta than Omicron. But infected people who had received two doses of vaccine were almost twice as likely to have Omicron from Delta, and those who had received three doses were three times more likely to have Delta.
This suggests that the Omicron variant is much better than the Delta in breaking down the immunity provided by vaccines. However, the team says a third booster vaccine still reduces the risk of Omicron infection by 50%. The findings have not yet been evaluated by peers.
Unpublished work from Grubaugh's lab suggests that the virus that circulates in the bodies of vaccinated people has less genetic diversity than that of non-vaccinated people, which means that vaccines do not promote new mutations. But as more people acquire immunity through infection or vaccination, viral strains that can avoid the immune response will have a growing advantage over others.
Nathaniel Landau, a microbiologist at New York University's Grossman Medical School, says the work supports the idea that a special Omicron vaccine would be useful, although he adds that there is always the possibility that a very different strain could dominate in the future. A key question that remains, says Landau, is whether something about the biology of the virus makes it more contagious, as well as more capable of bypassing the immune system. For example, the tendency of the virus to live in the upper airways instead of the lungs could make it more easily spread across the Delta.
Vaughn Cooper, an evolutionary biologist at the University of Pittsburgh in Pennsylvania, adds that the function of the mutations in the Omicron's protein was a particularly troubling question. "We do not understand how this strange combination of mutations found in the Omicron spike interacts to escape immunity," he says. "The huge number of new mutations makes it difficult to identify which combination is the cause."