Nearly 20 million lives were saved by COVID-19 vaccines during their first year, but even more deaths could have been prevented if international targets for the shots had been reached, researchers have reported.

On December 8, 2020, a retired shop clerk in England received the first shot in what would become a global vaccination campaign.

Over the next 12 months, more than 4.3 billion people around the world lined up for the vaccines.

The effort, though marred by persisting inequities, prevented deaths on an unimaginable scale, Oliver Watson of Imperial College London, who led the new modelling study, said.

"Catastrophic would be the first word that comes to mind," Watson said of the outcome if vaccines had not been available to fight the coronavirus.

The findings "quantify just how much worse the pandemic could have been if we did not have these vaccines".

Researchers used data from 185 countries to estimate that vaccines prevented 4.2 million COVID-19 deaths in India, 1.9 million in the United States, one million in Brazil, 631,000 in France and 507,000 in the United Kingdom.

An additional 600,000 deaths would have been prevented if the World Health Organisation target of 40 per cent vaccination coverage by the end of 2021 had been met, according to the study published on Thursday in the journal Lancet Infectious Diseases.

The main finding - that 19.8 million COVID-19 deaths were prevented - is based on estimates of how many more deaths than usual occurred during the time period.

Using only reported COVID-19 deaths, the same model yielded 14.4 million fatalities averted by vaccines.

The London scientists excluded China because of uncertainty around the pandemic's effect on deaths there and its huge population.

The study has other limitations.

Researchers did not include how the virus might have mutated differently in the absence of vaccines, and they did not factor in how lockdowns or mask wearing might have changed if vaccines were not available.

Another modelling group used a different approach to estimate 16.3 million COVID-19 deaths were averted by vaccines.

That work, by the Institute for Health Metrics and Evaluation in Seattle, has not been published.

"We may disagree on the number as scientists, but we all agree that COVID vaccines saved lots of lives," the institute's Ali Mokdad said.

The findings underscore the achievements and the shortcomings of the vaccination campaign, said Adam Finn of Bristol Medical School in England, who like Mokdad was not involved in the study.

"Although we did pretty well this time - we saved millions and millions of lives - we could have done better and we should do better in the future," Finn said.

Funding for the research came from several groups including the WHO; the UK Medical Research Council; Gavi, the Vaccine Alliance; and the Bill and Melinda Gates Foundation.