A growing number of countries are looking at switching to different COVID-19 vaccines for second doses, a measure particularly necessary in Denmark after health authorities discontinued inoculations with AstraZeneca's vaccine in April over rare side-effect concerns.
More than 144,000 Danes, mostly front-line personnel in the health sector and the elderly, received their first jab with AstraZeneca's vaccine but were subsequently vaccinated with either Pfizer-BioNTech or Moderna's shots.
"The study shows that fourteen days after a combined vaccination program, the risk of infection with SARS-CoV-2 is reduced by 88% compared to unvaccinated individuals," the State Serum Institute (SSI) said.
That is a "high efficacy", SSI added, comparable to the 90% efficacy rate of two doses from Pfizer-BioNTech's vaccine, confirmed in a different Danish study.
The study, published last week, covered a span of more than five months between February and June this year, a period in which the Alpha-variant of the coronavirus was predominant.
It could not conclude whether the same protection applied to the Delta-variant, which is now the most widespread in Denmark.
It also provided no efficacy data on COVID-19 related deaths or hospitalizations, since none took place following the combined vaccination program.