Patients who have recently taken the Covid-19 vaccine are advised to reschedule their yearly mammogram screenings, as the presence of swollen lymph nodes - one of the known side effects of the jab - could be mistakenly identified as a sign of breast cancer.
Doctors have advised patients to move their mammogram screenings to either before the Covid-19 jab or a few weeks after the second dose of the vaccine, to avoid confusion.
The Pfizer-BioNTech shot can cause swollen lymph nodes in the neck or arms, though this usually gets better by itself in a week or so, says the Ministry of Health.
Dr Tan Yah Yuen, a breast surgeon at Mount Elizabeth Hospital, said that recent Covid-19 vaccination may cause swollen underarm lymph nodes in the vaccinated arm, which could lead doctors to "falsely suspect" an early sign of cancer.
"If the clinical suspicion is that the swollen lymph node is due to the vaccination and not breast cancer, then a repeat ultrasound can be done in two to three months to follow up on the swollen lymph nodes for resolution," she said.
So far, she has seen two patients, both healthcare workers, who had their mammogram and ultrasound screenings after receiving their Covid-19 jabs. Both showed enlarged underarm lymph nodes.
Since there were no other "suspicious signs" for breast cancer, the hospital will follow up on these cases with a repeat ultrasound.
All other patients who just received their Covid-19 shots have been told to reschedule their check-ups, said Dr Tan, noting that The American Society of Breast Surgeons has recommended that women should consider scheduling mammogram or ultrasound checks before taking the first vaccine dose, or four to six weeks after the second dose.
Women who are in cancer remission should also discuss with their doctors the appropriateness of rescheduling their mammogram or ultrasound checks, and should seek to have their vaccine jab on the opposite arm if possible, to avoid false alarms of cancer recurrence, she advised.
Dr Ong Kong Wee, medical director of K.W. Ong Breast and General Surgery Clinic, said the concern over enlarged or swollen underarm lymph nodes is that it could be a sign of breast cancer spreading to the lymph nodes.
But it could also be due to infection or autoimmune diseases.
Dr Ong advised patients to schedule their mammogram screenings before taking the vaccine as this would prevent them from being subjected to more extensive and invasive tests, such as a needle biopsy.
Agreeing, infectious diseases expert Paul Tambyah, president of the Asia-Pacific Society of Clinical Microbiology and Infection, said that it would be prudent to schedule mammograms at least two weeks after receiving the vaccine, or prior to the vaccine, to avoid unnecessary anxiety.
He added that the Pfizer-BioNTech vaccine trial found that there were 64 cases of swollen lymph nodes among the vaccine recipients, versus six cases among those who had received the placebo. Both groups had more than 20,000 people each.
The report from the United States Centres for Disease Control and Prevention on the international trial had also stated that the lymph node swelling lasted an average of 10 days, said Professor Tambyah.
However, he noted that the trial recipients were not specifically asked if they had experienced lymph node swelling, so the figures from the trial might be under-reported.
On the other hand, results from the Moderna vaccine trial conducted in the United States found that 14 per cent, or 2,090, of the 14,677 vaccine recipients had developed swollen lymph nodes, against 3.9 per cent of the placebo recipients. The swelling typically lasted for around seven days after the vaccination.