The widespread use of vaccine campaigns around the world has greatly slowed the spread of SARS-CoV-2 where it is available, but limited supply is still a burden to be alleviated, especially in poorer countries.
Several vaccine distribution strategies have been proposed, and many studies have shown that vaccinating the most vulnerable first means achieving the highest survival rate or inserting those most likely to be transmitted and achieving the highest virus clearance. .
Gandon & Lion (June 8) on paper recently uploaded to medRxiv * prepress serverth, 2021) evaluates the impact of some vaccine strategies on the likelihood of causing mutants to escape the SARS-CoV-2 vaccine, which may play an increasingly important role in future COVID-19 management.
Vaccine results. Image Credit: https://www.medrxiv.org/content/10.1101/2021.06.09.21258644v1.full.pdf
Vaccination against transmission
The authors developed a selection coefficient, a quantitative description of the relative suitability of a group, applied to the probability of developing vaccine escape mutants. This value took into account the quality of the host, representing an individual who was not vaccinated or not vaccinated, and allowed them to track the frequency of mutant escapes that were developing.
Two types of hosts were first constructed: those with low or high levels of social interaction, representing the fewest and most transmissive individuals in a population. In addition, a value was applied to the chance of escaping immunity caused by a vaccine mutant, from 0 to 1, and to the escape mutant that develops in an inserted host.
According to the formula developed by the group, having a higher relationship rate between individuals results in a more intensive selection of SARS-CoV-2 mutants, just as vaccination becomes more likely to develop mutant escapes. Therefore, in an vaccination campaign aimed primarily at high-contact people, mutant escape reduced the chance of developing stronger, although targeting low-contact people, who are often vulnerable, greatly reduces mortality.
The vaccine strategy of distributing one dose more broadly to each individual, rather than providing more than two individual doses, was investigated by the team by introducing a “partially embedded” category. This strategy results in less selection pressure among those who are fully vaccinated, as there are fewer, although a larger number of partially vaccinated people promote the effect that escape mutants develop even more strongly.
The group found that the latter effect was stronger and that the frequency of mutant escapes increased according to the single-dose vaccination strategy, although in this case this strategy also decreased the mortality rate because the larger proportion of the population was at least partially protected and beneficial. from the slightest symptoms of COVID-19 produced during vaccination.
Future vaccination strategies
According to the group, these findings require more robust research before applying any specific recommendations for ongoing vaccination campaigns, but these findings seem likely due to differences in behavior (high or low contact) and vaccine status (fully, partially). or those who do not have the vaccine), can have a significant impact on the development of escape mutants that can greatly shorten the useful life of the vaccine.
Other studies that have examined the selection pressure for SARS-CoV-2 among hosts have concluded that, compared to the previous one, the insertion of the most vulnerable first increases the frequency of escape mutations. However, this study focused on internal host selection pressure to better reflect the selection of escape mutants from embedded individuals, and the authors noted that the combination of inter-host and intra-host selection factors remains to be studied.
The duration of any vaccine that can be used may be at stake if the frequency of escape is not taken into account in future vaccination campaigns, it may create a constant battle with COVID-19 that requires constant updating of vaccines.
* Important note
medRxiv publishes advanced scientific reports that are not reviewed and therefore should not be considered decisive, should be directed to clinical practice / health-related behavior or treated as established information.
- Gandon, S. and Lion, S. (2021). Targeted vaccine and SARS-CoV-2 adaptation rate.