Issues and Experts

To delay or not to delay a second dose? Economics pre-print study examines effectiveness of vaccine strategy

September 21, 2021

New research from Simon Fraser University suggests extending the interval between the first and second COVID-19 vaccine doses in the U.K. and Canada saved lives.

The team found that this vaccine strategy effectively reduced new infections and deaths in countries facing mRNA and AstraZeneca vaccine supply constraints.

“Canada’s decision on delaying the second dose was indeed a smart move. In our counterfactual experiment where the dosing interval is eight weeks, it is shown that Canada would have had substantially more cases and deaths,” says study lead author and SFU economics assistant professor Dongwoo Kim.

In their new pre-print study, published in arXiv, an open-access repository supported by Cornell University, researchers analyzed country level epidemiological data from the Our World in Data project over a 13-month period from June 1, 2020 to July 8, 2021. More than 40 countries, European Union and OECD member countries, Uruguay, UAE and Bahrain were included in the analysis.

They also considered government policy responses using the Containment and Health Index developed by the Oxford COVID-19 Government Response Tracker (OxCGRT) and people’s behavioural changes using country level data from Google COVID-19 Community Mobility Reports.

Impact of delaying doses:

Vaccine manufacturers had recommended an interval of three to four weeks for mRNA vaccines Pfizer-BioNTech (Comirnaty) and Moderna (SpikeVax) and an eight- to-12-week interval for the AstraZeneca-Oxford (Vaxzevria) vaccine.

However, supply constraints and rising cases of infection prompted health officials in the U.K. and Canada to extend the interval between the first and second doses up to 12 to 16 weeks to maximize the number of people with partial protection against the virus.  In contrast, the U.S. and Israel did not face the same supply constraints and were able to follow the recommended dosing interval.

The researchers found that progress in first dose vaccination significantly reduced weekly growth rates of new cases and deaths. Conditioning on first dose progress, full vaccination offered no further reductions. The impact of vaccination was larger for countries that extended the dosing interval.

On the other hand, the researchers found that delaying the interval between the first and second doses was not effective when vaccines developed by Chinese firms (Sinovac and Sinopharm) were used, as was the case in some developing countries such as Bahrain, Chile, Turkey, UAE and Uruguay.

Key takeaways:

Researchers suggest middle and low-income countries which are still faced with a shortage of vaccines could delay the interval between the first and second doses of an mRNA or AstraZeneca vaccine to protect more people from worse outcomes, hospitalizations and deaths.

However, if relying on Chinese-developed vaccines, countries may need to administer both shots to provide protection against infection to the population.

The researchers also noted that vaccination progress increases people’s mobility, which indirectly leads to higher growth rates of new infections and deaths.

The researchers conclude that public health measures are still a key to containing the spread of the virus by restraining people’s mobility and social interactions after they are vaccinated.


DONGWOO KIM, assistant professor, economics                     


MELISSA SHAW, SFU  Communications & Marketing 
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