Delayed Covid-19 doses could cut mortality rates, US study finds

For people under 65, a delay strategy could mean fewer deaths

There are circumstances when people may benefit from a delayed dose, according to a US study published in 'The BMJ'. AFP

Delaying the second dose of Covid-19 vaccines could in some conditions reduce mortality by up to 20 per cent, research published on Wednesday shows.

The vaccines developed by Pfizer-BioNTech and Moderna are both two-dose treatments but there are circumstances when people may benefit from a delayed dose, according to a US study published in The BMJ on Wednesday.

For people under 65, the circumstances include having a one-dose vaccine efficacy of 80 per cent or higher and vaccination rates of 0.1 to 0.3 per cent of the population a day.

“Decision-makers will need to consider their local vaccination rates and weigh the benefits of increasing these rates by delaying a second dose, versus the risks associated with the remaining uncertainty in this strategy," the researchers said.

"These decisions should continue to be re-evaluated as new data become available."

The longer it takes to effectively vaccinate the world, the higher the chance that variant strains will develop with immunity to available drugs.

This has sparked calls for giving priority to single-dose vaccines.

But the assumption that meaningful protection against Covid-19 can be achieved after a single dose of vaccine is the subject of continuing debate.

The US team of researchers set out to measure the effect of delayed second dose vaccine policies on infections, hospital admissions and deaths, compared with the current on-schedule, two-dose regimen.

Using a simulation model based on a sample population of 100,000 US adults, they ran scenarios to forecast potentially infectious interactions under different conditions over a six months.

It included varying levels of vaccine efficacy and administration rates, as well as varying assumptions as to whether the vaccine prevents transmission and serious symptoms or only prevents serious symptoms, including death.

They also examined the effect of delaying second doses for those younger than 65, but not before fully vaccinating older people.

The results suggest that under specific conditions, a decrease in cumulative mortality, infections and hospital admissions can be achieved when the second vaccine dose is delayed.

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