There have been four cases of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the lining outside the heart) among men aged 18 to 30, following the second dose of a Covid-19 vaccine in Singapore. This is out of a total of six such cases reported here, the authorities said.

As a precaution, the Republic's expert committee on Covid-19 vaccination said that vaccinated people, especially adolescents and younger men, should avoid strenuous physical activity for one week after their second vaccine dose.

During this period, they should seek medical attention if they develop chest pain, shortness of breath or abnormal heartbeats, said the expert committee in a statement last night.

"Most cases are mild, recover without the need for significant intervention and do not suffer any long-term effects, although very rarely, severe cases may result in damage to the heart muscles," said the expert committee.

"Our assessment is that the benefits of receiving the mRNA Covid-19 vaccines, that is, the reduction in Covid-19 infections and severe complications even if infected, continue to outweigh the risks of vaccination."

The four cases reported by the Health Sciences Authority (HSA) are aged between 18 and 30, and had their symptoms a few days after receiving the second dose of an mRNA-based vaccine.

Both Covid-19 vaccines in Singapore - by Pfizer-BioNTech and Moderna - use mRNA technology.

All four men have recovered or been discharged from hospital.

The expert committee said that the four cases are at "the upper end of the expected range of this age group, based on background incidence rates".

"While further studies and investigations are ongoing, the currently available data suggests that there may be a very small risk of myocarditis and pericarditis after the second dose of an mRNA vaccine, particularly in young men," it said.

The expert committee also said that heart inflammation following the second dose of the vaccine has been observed in Israel and the United States in men below 25.

The risk is estimated to be at 1.6 cases per 100,000 doses in the US.

The expert committee said there has been no increased risk associated with the first dose of the vaccine, and that the condition is generally more common in men than in women.

Last month, it was reported that 20 cases of anaphylaxis - a severe, life-threatening allergic reaction - had happened in Singapore in relation to Covid-19 vaccination.

In a separate update, the HSA said cases of myocarditis and pericarditis have been reported with Covid-19 vaccination both overseas and locally.

Myocarditis and pericarditis have many causes - most are due to viral infections and immunological reactions, it said, and in most cases, the inflammation is mild.

The authority said it has received six reports of myocarditis and pericarditis, of which four were in men younger than 30 years.

"HSA will continue to monitor this potential safety issue closely," it said.

Ng Wei Kai