Finland on Thursday joined Sweden, Denmark and Norway in recommending against use of Moderna Inc.’s Covid-19 vaccine in younger age groups, citing risks of rare cardiovascular side effects they said warranted the precautionary steps.

Finland’s Institute for Health and Welfare said Thursday it would pause use of the Moderna vaccine among men under the age of 30, following a similar step Wednesday by Swedish regulators. Denmark on Wednesday said it wouldn’t offer the Moderna vaccine to under-18s as a precautionary measure.

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Finland on Thursday joined Sweden, Denmark and Norway in recommending against use of Moderna Inc.’s Covid-19 vaccine in younger age groups, citing risks of rare cardiovascular side effects they said warranted the precautionary steps.

Finland’s Institute for Health and Welfare said Thursday it would pause use of the Moderna vaccine among men under the age of 30, following a similar step Wednesday by Swedish regulators. Denmark on Wednesday said it wouldn’t offer the Moderna vaccine to under-18s as a precautionary measure.

Norway on Wednesday advised that all under-18s shouldn’t be given the Moderna vaccine, even if they had already received one dose, and recommended that men under 30 consider getting the vaccine developed by Pfizer Inc. and BioNTech SE instead. Norwegian officials cited U.S., Canadian and Nordic data, saying the absolute risks remain low and calling the advice “a precautionary measure.”

The European Medicines Agency said Thursday that new preliminary data from the Nordic countries supports a warning the agency adopted in July that inflammatory heart conditions called myocarditis and pericarditis can occur in very rare cases following vaccination with Covid-19 shots made by Moderna and Pfizer-BioNTech.

Both vaccines both use a new technology called messenger RNA. European and other data show instances of rare cardiovascular side effects occurring more frequently after Moderna vaccinations than after Pfizer.

A Moderna spokesman said Thursday that the company is aware of the European countries’ decisions and “the very rare occurrence of myocarditis and/or pericarditis following administration of mRNA vaccines against Covid-19…These are typically mild cases and individuals tend to recover within a short time following standard treatment and rest.”

The Moderna spokesman added that Covid-19 itself increases the risk of myocarditis “and vaccination is the best way to protect against this.”

The rare instances occur more often after the second dose of either vaccine, and in younger men, the EMA said. It continued to describe the vaccines as safe and effective against severe Covid-19 illness and death. The EMA’s safety committee “will assess the new data to determine whether there is a need to update the current advice in the product information for the vaccines,” a spokeswoman said Thursday.

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Individual European countries can decide alone to limit vaccines’ use based on their assessments of risk factors including infection rates within their own populations.

The European decisions follow a recommendation by Ontario health regulators in late September that young people ages 12 to 24 receive a Pfizer vaccine instead of Moderna. Ontario’s Health Ministry said the advice was based on an increased number of reports of pericarditis and myocarditis following vaccination with Moderna, compared with Pfizer.

The EMA said Thursday that further evaluation is needed of data suggesting the risk of myocarditis could be greater after a second dose of Moderna compared with Pfizer.

Advisers to the U.S. Centers for Disease Control and Prevention in June said there was a “likely association” between the Moderna and Pfizer-BioNTech vaccines and very rare inflammatory heart conditions in some younger people who received them, but that most patients recovered quickly.

In September, the rare cardiovascular conditions also factored into U.S. regulators’ discussions about booster shots. Advisers to the Food and Drug Administration voiced concerns over the limited data about whether a third dose of the Pfizer-BioNTech vaccine increased the risk of myocarditis. FDA officials told panel members that the risk of the condition from a third dose was unknown and more research was needed.

The EMA extended its recommendation for use of the Moderna vaccine in July to children aged 12 and older.

Write to Jenny Strasburg at jenny.strasburg@wsj.com and Dominic Chopping at dominic.chopping@wsj.com