An unnamed woman has died of myocarditis in New Zealand after receiving the Pfizer COVID-19 vaccine shots. Myocarditis is an inflammation of the muscles surrounding the heart. Although the government has linked the woman’s death to the vaccination, an investigation into her death has commenced knowing exactly what happened, Reuters reports.

The country’s Independent Vaccine Safety Monitoring Board said the Pfizer vaccine could have provoked myocarditis, but other health factors could have also played a role in the woman’s death. The Ministry of Health said an autopsy will be conducted to determine the exact cause of her death.

“This is the first case in New Zealand where a death in the days following vaccination has been linked to the Pfizer COVID-19 vaccine,” the health ministry said. “While the Centre for Adverse Reactions Monitoring has received other reports of deaths in someone recently vaccinated, none are considered related to vaccination.”

According to the European Medicines Agency (EMA), myocarditis can result as a very rare side effect of COVID-19 vaccines. People who experience the symptoms often recover with little medical attention. Some signs of myocarditis include abnormal heartbeat, shortness of breath, and mild chest pain. The agency however stated that the health benefits of vaccines far outweigh any potential risks.

Pfizer said any adverse reactions to its vaccines are extremely rare, and that it continues to monitor recipients for unexpected reactions.

“Pfizer takes adverse events that are potentially associated with our vaccine very seriously,” the drugmaker said. “We closely monitor all such events and collect relevant information to share with worldwide regulatory authorities.”

While the inflammation of the muscular tissues of the heart is rare, health experts said it occurs to men more than it does to women. Government officials said there are 3,465 cases of COVID-19 in New Zealand with 26 related deaths, and that 23% of the population is fully vaccinated with more than three million doses of either Pfizer or AstraZeneca COVID-19 vaccines. New Zealand has a population of about 5 million people.