SINGAPORE - Swimming, cycling, lifting heavy weights, as well as ball and racket games are some of the activities individuals should avoid for seven days after receiving their first and second Covid-19 jabs, said the Ministry of Health (MOH).

The clarification in an MOH advisory on Monday (July 5) followed an amended recommendation that people should avoid strenuous physical activity after either of their mRNA Covid-19 vaccine doses.

An earlier advisory on June 11 had recommended that those vaccinated should avoid strenuous physical activity for one week after their second dose of the mRNA Covid-19 vaccines.

The latest advisory says competitive sports and physical education classes should also be avoided as they are considered strenuous.

Physical activities safe to do one week post-vaccination include casual walking, stretching, working while standing, and housework.

"We recommend that everyone, in particular adolescents and younger men, aged younger than 30, avoid strenuous physical activity, such as intense exercise, for one week after the first and second doses," according to the latest MOH advisory.

"During this time, if you develop any chest pain, shortness of breath or abnormal heart beats, you should seek medical attention immediately."

The ministry said that it updated its recommendation based on new data here and internationally.

MOH said in a press release on Monday that a 16-year-old male suffered a cardiac arrest shortly after a strenuous weight-lifting session at a gym six days after he received his first dose of an mRNA Covid-19 vaccine.

Investigations are ongoing.

"While most persons with vaccine-related myocarditis observed locally and internationally have mild symptoms and make an uneventful recovery, it is possible that the condition may be aggravated by factors or strenuous activities that may affect the heart," MOH said on Monday.

The HSA safety update also showed that seven of the 12 cases of myocarditis and pericarditis that occurred in individuals after they took the mRNA vaccines here were aged below 30. This is higher than expected for this age group.

The two inflammatory conditions occur more often in men than in women.

Cardiologists contacted by The Straits Times on Tuesday agreed it was wise to refrain from even moderate exercise for at least a week after vaccination.

Moderate exercise can include weight training, brisk walking, slow jogging (5kmh to 6kmh), cycling or swimming.

"Given the new knowledge of the risk of myocarditis occurring in young males after Covid-19 vaccination, it is prudent that exercises, particularly those of moderate to high intensity, be withheld for a week," said Professor Tan Huay Cheem, a senior consultant at the Department of Cardiology at the National University Heart Centre, Singapore.

"This is because we are unable to predict who will develop myocarditis after vaccination, even if the incidence is low."

High-intensity exercise includes circuit training, vigorous forms of weight training, and moderate activities done at a heart-pounding pace, such as sprinting or swimming longer distances, he said.

Prof Tan said this precaution should apply to all individuals, but especially younger people, who tend to be more active and likely to engage in strenuous activity.

"As a general guide, strenuous exercise will be an activity in which you are unable to speak in complete sentences during the workout," he said. "If you can walk and sing while exercising without feeling out of breath, then it is low-intensity exercise."

He said that high-functioning athletes who are concerned about losing their conditioning can consider "downgrading their level of exercise" to a low intensity.

Dr Kenneth Ng, a consultant cardiologist at Novena Heart Centre, thinks it is prudent to "take a complete rest from exercise for one week to 10 days after vaccination".

After that, one can progressively increase activity, he said.

Correction note: An earlier version of this story said that the Health Sciences Authority reported about the 16-year-old case in its Covid-19 vaccine safety update. The case was, in fact, reported by MOH in a press release on Monday. We are sorry for the error.