High vaccination coverage cuts COVID-19 death rate by 80 per cent: US study

Death rates were over 80 per cent lower in communities with high vaccination coverage, according to a large US study published in The BMJ.

The observational study, based on data across 2,558 counties in 48 US states, show that counties with better vaccination significantly lowered death rates compared with largely unvaccinated counties.

This large benefit complements the growing body of evidence indicating individual level benefits of COVID-19 vaccination, the researchers said.

As of April 11, 2022, over 11 billion COVID-19 vaccine doses have been administered globally and the World Health Organization's target is to vaccinate 70 per cent of the world's population by mid-2022.

While previous vaccine studies have shown benefits at the individual level, the wider population level impact of scaling up COVID-19 vaccination remains largely unknown.

Researchers at the US Centers for Disease Control and Prevention (CDC) set out to estimate how increasing county coverage of vaccines affected population level mortality and incidence of COVID-19.

The findings are based on more than 30 million cases of COVID-19 and over 400,000 deaths linked to COVID-19 across 2,558 US counties, which were reported during the second year of the pandemic, between December 2020 and December 2021.

The researchers measured effectiveness by comparing reported COVID-19 incidence and mortality rates in counties with very low (0-9 per cent), low (10-39 per cent), medium (40-69 per cent), and high (70 per cent or more) vaccination coverage.

Vaccination status was defined as the percentage of adults aged 18 and over who had received at least one dose of a COVID-19 preventive.

After taking account of potentially influential factors, the researchers found that increased vaccination coverage in counties was associated with reduced levels of COVID-19 related mortality and cases.

For example, during the first half of 2021, when the Alpha variant of coronavirus was dominant, the COVID-19 mortality rate was reduced by 60 per cent, 75 per cent, and 81 per cent in counties with low, medium, and high vaccination coverage, respectively, compared with those that had very low coverage.

The corresponding figures for the reduction in cases were 57 per cent, 70 per cent, and 80 per cent, the researchers said.

Similar reductions in mortality were also seen during the second half of 2021 when the Delta variant became dominant in the US, although with smaller effects on case levels, they said.

"The research adds to the evidence that vaccination can prevent infection and illness on a large scale," said Professor Christopher Dye from the University of Oxford, UK, in a linked editorial.

"The findings of this study also make clear that many more lives could have been saved, and will be saved, by encouraging people to keep up to date with vaccination in the face of waning immunity and new coronavirus variants and by achieving even higher population coverage," he added.

This is an observational study, so cannot establish cause, the researchers said.

They also acknowledged several limitations should be considered when interpreting these data.

For example, additional markers of severe disease, such as hospital admissions, were not explored and they did not control for factors such as rules on wearing a face mask and physical distancing at the time, which may have affected their results.