Finland now recommends that people under 65 who received a first dose of AstraZeneca’s coronavirus vector-based vaccine to have their second shot be an mRNA-based vaccine.
Other countries, including Germany, France, Norway and Denmark have made similar decisions, due to concerns over rare blood clot side effects from the AstraZeneca jab.
The policy of a 12-week period between doses would be the same for individuals whose first shot was AstraZeneca, according to the Institute for Health and Welfare (THL).
So far, around 100,000 people in Finland — mainly individuals belonging to medical risk groups — received an AstraZeneca jab before news of rare side effects emerged, according to THL's chief vaccinologist Hanna Nohynek.
She noted that the UK will in the coming weeks conclude initial mix-and-match vaccine trials where individuals who first received a dose of the AstraZeneca vaccine were given boosters from Pfizer-Biontech, which is an mRNA vaccine.
THL: Combining vaccines ok
Nohynek noted that from a biological standpoint alternating types of jabs should not be an issue as Covid-19 vaccines train the immune system to respond to the virus' spike protein.
"Adverse effects can emerge if the immune system overreacts, causing the vaccines to boost each other and trigger an inflammatory response. But we're hoping two different vaccines actually provide even stronger immunity to the virus," she explained.
Nohynek told Yle that in hindsight it would have been wise for health authorities to have studied the effects of mix-and-match regimens earlier.
She said it remained unclear how many additional Covid booster shots people will need in the future.
"It will soon be a year since the first people received a Covid vaccine, so studies will look at how well the vaccine has protected them from the disease. Future vaccination plans also depend on how much the original strain of the virus mutates. The UK plans to roll out third shots for autumn booster jabs in conjunction with seasonal flu shots," she explained.
Going forward, Finland's stockpile of AstraZeneca vaccines will now be dedicated to inoculating people over the age of 65.