Story at a glance

  • Researchers measured the health outcomes of individuals who waited more than three weeks to get their second shot.
  • For vaccines with an efficacy rate of more than 70 percent, cumulative mortalities fell.
  • This could help countries struggling to rollout the vaccines in a timely manner.

Delaying the second dosage of the COVID-19 vaccine could result in a lower COVID-19 mortality rate, according to a new study published in the British Medical Journal

In the United States, the vaccination rate has been consistently increasing under President Biden’s ambitious inoculation goals. But other countries are experiencing more delay in vaccinating their residents.

Public health experts have advised that Pfizer vaccines be given three weeks apart and Moderna vaccines be given four weeks apart for optimal efficacy. 

Now, new research suggests that waiting on the second dose may pose some public health benefits. 

A team of international scientists and researchers constructed a simulation examining the health outcomes associated with the standard interval between COVID-19 vaccinations versus those associated with a delayed second shot.

They found that the cumulative mortality rate tended to be lower in the simulation where participants delayed their second shot beyond the recommended 21 to 28 days after the first shot.


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Results displayed up to 20 percent less mortality, namely for hypothetical countries that have a lower vaccination rate at 0.1 or 0.3 percent.  

The report authors made a point to exclude people aged 65 years and older who tend to be more vulnerable to severe COVID-19 infections. 

“A delayed second dose vaccination strategy, at least for people aged under 65, could result in reduced cumulative mortality under certain conditions,” the authors concluded.

Notably, researchers found that the declining mortality rate was primarily associated with situations with a low vaccination rate and when the vaccine used in the simulation is above 70 percent effective at preventing infections. 

“These results suggest that higher first dose efficacy estimates favor delaying the second dose and that for a first dose efficacy of 70% or below, no meaningful difference is apparent between the standard and delayed second dose strategy,” the report reads.

This could have beneficial implications for countries struggling to secure enough vaccine doses to vaccinate their population and that, in certain situations, delaying a second dose could help lower deaths due to COVID-19.

Other scientific literature has reached similar conclusions. Additional researchers have found that delaying the second vaccine is safe, although not always ideal, and could be used as a public health strategy given that the vaccines available have an efficacy rate of at least 80 percent. 

Additionally, a randomized clinical trial conducted by AstraZeneca showed little antibody decay three months following the first vaccination, implying a strong and relatively long-lasting immunity. 


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Published on May 13, 2021