As the number of vaccinated people increases, the competitive advantage of resistant strains increases. According to a new study, the risk is at its highest when vaccination coverage reaches 60%, which is exactly the threshold currently reached in the US.
The whole world is hoping that mass vaccination will stop the COVID-19 epidemic. However, a resistant variant to the vaccine must not be allowed to emerge. Still, the risk of developing a resistant strain is greatest when a large proportion of the population is vaccinated, but at a rate that is not enough to ensure herd immunity, according to a new study published in Nature Scientific Reports.
The researchers simulated the probability of a resistant strain in a population of 10 million over the next three years, taking into account the number of vaccinated people, the mutation rate of the virus, the rate of spread, and the successive “waves” of sharply increasing infections, followed by a decrease in new cases after restrictions are imposed (restrictions, closures, etc.). Not surprisingly, they concluded that rapid vaccination and low prevalence reduce the risk of a resistant variant emerging. However, the study also shows that the risk is highest when a large proportion of the population is vaccinated, but not enough to ensure herd immunity.
Selection pressure increases as vaccination coverage increases
When a large proportion of the population is vaccinated, especially the at-risk population (the elderly and sick), policymakers and individuals are tempted to revert to pre-pandemic guidelines and behaviors that can contribute to high levels of virus transmission. However, this is the most favorable time for the emergence of resistant variants because of the phenomenon of “selection pressure”: as the number of vaccinated people increases, the competitive advantage of vaccine-resistant strains increases. This is what has happened to the Delta variant: as the proportion of vaccinated people continued to increase, its infective capacity has had to increase tenfold to continue spreading at the same rate.
No Country has yet reached the 90% rate needed for herd immunity
The problem is that this 60% appears to be the ceiling rate that we see in most countries. No country in the world has yet managed to reach the threshold of 90% collective immunity. If the coronavirus epidemic, unfortunately, continues at this rate and herd immunity is not attained, it may be necessary to resort to compulsory vaccination.
The virus continues to circulate even among vaccinated populations
Last Friday, the US Centers for Disease Control and Prevention (CDC) warned that while the vaccine prevents the risk of serious illness, it does not completely prevent the risk of spreading the infection “because the immunity that is generated is not primarily mucosal” (it does not prevent the virus from developing in the nose). Therefore, the CDC recommended that even vaccinated persons should again wear a mask indoors.
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