HIGH VACCINATION RATES RAISE RISK OF VIRUS SPREAD, STUDY SHOWS

PARIS—Relaxing restrictions like mask-wearing and social distancing when most people have been vaccinated greatly increases the risk of vaccine-resistant variants of the virus that causes COVID-19, new research showed on Friday.

At a time when nearly 60 percent of Europeans have received at least one vaccine dose, the authors said their modeling study showed the need to maintain nonvaccination measures until everyone is fully jabbed.

To predict how the SARSCoV-2 virus might mutate in response to vaccination campaigns, a pan-European team of experts simulated the probability of a vaccine-resistant strain emerging in a population of 10 million people over three years.

Variables included vaccination, mutation and transmission rates—including recurring “waves” of infections and falls in cases in response to lockdowns.

Predictably, the model showed that a rapid rate of vaccination reduced the risk of a resistant strain emerging.

No herd immunity

But in a “counterintuitive result,” the model showed that the highest risk of resistant strains emerging came when a large part of the population was vaccinated, but not large enough to have herd immunity.

The authors said the model showed a threshold of 60 percent of the population vaccinated, after which resistant variants were more likely to occur.

The situation in the United States—where 60 percent of adults are fully vaccinated and 80 percent of new cases are caused by the Delta variant—is similar.

“Vaccines are our best bet to beat this pandemic” said coauthor Simon Rella, from Austria’s Institute of Science and Technology.

Strain advantage

“What our model showed is that when most people are vaccinated, the vaccine-resistant strain has an advantage over the original strain.

“This means that the vaccine-resistant strain spreads through the population faster than the original strain at a time when most people are vaccinated,” Rella said.

Viruses constantly mutate in response to constraints, such as growing immunity and measures designed to limit spread.

COVID-19’s Delta variant is significantly more infectious than the original virus, but there is currently little evidence to suggest it is more deadly.

Authors said that their research highlighted the need to maintain other anti-COVID measures until everyone is vaccinated. “We urge caution,” said Fyodor Kondrashov, a study coauthor.

“Evolution is a very powerful force and maintaining some reasonable precautions throughout the whole vaccination period may actually be a good tool to control this evolution.”