A team of US researchers has found that delaying the second dose of Covid-19 vaccines, at least for people aged under 65, could result in up to 20 percent lower mortality, but only under certain conditions.

These conditions include one dose vaccine effectiveness (efficacy) of 80 percent or higher and vaccination rates of 0.1 percent to 0.3 percent of the population per day, according to the study, published by The BMJ. If these conditions apply, the strategy could prevent between 47 and 26 deaths per 100,000 people, respectively, the study noted.

"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," write researchers including Thomas C. Kingsley, Assistant Professor of Medicine at Mayo Clinic, Minnesota.

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

Both the Pfizer and Moderna covid-19 vaccines in a standard two dose schedule are highly effective at preventing symptomatic infections and death. But immunity worldwide remains low, partly owing to low vaccination rates.

The longer it takes to effectively vaccinate the global population, the greater the likely risk of vaccine resistant strains developing. This has led to calls to prioritise single-dose vaccination for as many people as possible, even if this means delaying a second dose beyond the studied time frame.

"The justification for this relies on the assumption that meaningful protection against Covid-19 can be achieved after a single dose of vaccine, but this is the subject of intense debate," the team said.

To explore this further, they set out to measure the impact 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 "real-world" sample population of 100,000 US adults, the team ran a series of scenarios over a six-month period.

The results suggest that a delayed second dose strategy is optimal for vaccination rates at or lower than 0.3 per cent population per day if the vaccine efficacy from one dose is 80 per cent or greater.