Vaccine stockpiling by nations may strongly impact global trajectories of COVID-19 case numbers, and increase the potential emergence of novel variants of the SARS-CoV-2 virus, a modelling study has warned.

The research, published in the journal Science on Tuesday, explored the effects of different vaccine-sharing schemes on the global persistence of COVID-19 infections, as well as the possibility for the evolution of new variants, using mathematical models.

The researchers noted that allocation of COVID-19 preventives between countries has so far tended towards vaccine nationalism, wherein countries stockpile vaccines to prioritise access for their citizenry over equitable vaccine sharing.

"Certain countries that have had severe COVID-19 outbreaks have received few vaccines, while many doses have gone to countries experiencing comparatively milder pandemic impacts, either in terms of mortality or economic dislocation," said study co-first author Caroline Wagner, an assistant professor at McGill University in Canada.

"As expected, we have seen large decreases in case numbers in many regions with high vaccine access, yet infections are resurging in areas with low availability," study co-first author Chadi Saad-Roy, a graduate student at Princeton University in the US, added.

The researchers projected the incidence of COVID-19 cases under a range of vaccine dosing regimes, vaccination rates, and assumptions related to immune responses.

They did so in two model regions: One with high access to vaccines -- a high-access region (HAR) -- and a low-access region (LAR).

The models also allowed for the regions to be coupled either through case importation, or the evolution of a novel variant in one of the regions.

The study found that increased vaccine-sharing resulted in reduced case numbers in LARs.

"Because it appears that vaccines are highly effective at reducing the clinical severity of infections, the public health implications of these reductions are very significant," said study co-author Michael Mina, an assistant professor at the Harvard T H Chan School of Public Health, US.

Senior study author C Jessica Metcalf, a Princeton associate professor, noted that high case numbers in unvaccinated populations will likely be associated with higher numbers of hospitalisations and larger clinical burdens compared to highly vaccinated populations.

The authors also drew on a framework developed in their previous work to quantify the potential for viral evolution under different vaccine sharing schemes.

In their model, repeat infections in individuals with partial immunity -- either from an earlier infection or a vaccine -- resulted in the evolution of novel variants.

"Overall, the models predict that sustained elevated case numbers in LARs with limited vaccine availability will result in a high potential for viral evolution," said study senior author Bryan Grenfell, an associated faculty member in Princeton's High Meadows Environmental Institute (HMEI).

"The study strongly underlines how important rapid, equitable global vaccine distribution is," Grenfell said.

The researchers noted that in a scenario where secondary infections in individuals who have previously been infected strongly contribute to viral evolution, unequal vaccine allocation appears particularly problematic.

Global vaccine coverage will reduce the clinical burden from novel variants, while also decreasing the likelihood that these variants emerge, they added.