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Breast Cancer Now has stressed women should not delay routine mammograms after their coronavirus vaccine.
Temporary swelling of the lymph nodes in the armpit area, which contains breast tissue, is a relatively common side effect of any jab. This is due to immune cells gathering in the glands, which also occurs while naturally fighting off an infection.
In early February 2021, medics from the Hospital of the University of Pennsylvania reported the US' approved vaccines – just Pfizer-BioNTech and Moderna at the time – were causing a "notably higher incidence" of armpit swelling than non-coronavirus jabs.
A team from Massachusetts General Hospital later warned the lymph node swelling may get picked up by routine mammograms, which could be confused for glands that have become inflamed due to a tumour.
Watch: Swollen lymph nodes after vaccination could lead to false breast cancer diagnosis, doctors say
A patient may even have an unnecessary biopsy to confirm whether they have the disease, added the team.
Nevertheless, experts have stressed women should not delay routine mammograms, with the medics carrying out the procedure usually able to differentiate between vaccine-related swelling and signs of cancer.
This comes on International Women's Day, which aims to "assist women to be in a position of power to make informed decisions about their health".
In the Pfizer-BioNTech trial that led to its UK and US approval, scientists noted 0.3% of those who had the vaccine and less than 0.1% given a placebo shot reported lymph node swelling, which generally resolved within 10 days.
Armpit swelling on the right side was deemed "serious" in just one Pfizer-BioNTech participant. The side effect "is likely to have resulted from a robust vaccine-elicited immune response".
The Moderna team also reported armpit "swelling/tenderness" as an adverse event in its clinical trial.
The UK has a third jab in its immunisation arsenal, University of Oxford-AstraZeneca, which did not throw up lymph node swelling concerns in the study that led to its approval.
"As with all medicines, the COVID-19 [the disease caused by the coronavirus] vaccine can cause side effects," Addie Mitchell, a clinical nurse specialist at Breast Cancer Now, told Yahoo UK.
"While some common side effects are nausea, fatigue and headache, one of the more uncommon side effects of the vaccine is enlarged lymph nodes, including in the armpit, which will usually go within two weeks.
"In the UK, no guidance has been issued about needing to delay mammograms due to lymph node swelling as a result of the COVID-19 vaccine and so we encourage women to attend their appointments when invited.
"We want to reassure women that in an X-ray, radiologists can usually distinguish lymph nodes in the breast and armpit area that are enlarged because they are cancerous from those that are enlarged for other reasons, including when this is a side effect of the vaccine.
"If you are aware you have had enlarged lymph nodes following your COVID-19 vaccine when you attend your breast screening, speak to the mammography practitioner."
If the swelling persists for several weeks, contact your GP.
"A lump or swelling in the breast, upper chest or armpit can be a sign of breast cancer and it's vital during the COVID-19 pandemic women still get in touch with their GP if they find any new or unusual breast changes, even if they have recently had the COVID-19 vaccine," said Mitchell.
"While most breast changes won't be cancer, on the occasions it is, early diagnosis increases the chance of successful treatment.
"If you need to attend an appointment there will be safety measures in place to reduce the risk of the COVID-19 infection spreading."
Scientists from Cardiff University recently reported more than two in five people with suspected cancer symptoms during the UK's first lockdown did not seek medical help.
"There is the worry delaying breast cancer screenings further, when the pandemic has had such a considerable impact on delaying appointments, could lead to late diagnosis and the inability to treat early," Dr Priyanka Vaidya, a GP at Pall Mall Medical, told Yahoo UK.
"For women with a personal or family history of breast cancer, or carriers of the BRACA gene, which increases your susceptibility to breast cancer, I would suggest engaging in a discussion with their GP around the likelihood of post-vaccination swelling".
Watch: Continue having mammograms during pandemic
One in seven women in the UK will statistically develop breast cancer at some point in her life.
Mammograms, an X-ray, help to spot tumours that are too small to see or feel.
The risk of developing breast cancer increases with age. In the UK, women who are registered with a GP are automatically invited for a breast cancer screening every three years from age 50 to their 71st birthday.
This screening is running amid the pandemic.
Scientists from Queen Mary University of London recently reported how women who skip even one routine mammogram before a breast cancer diagnosis face a significantly higher risk of dying with the disease.
"Regular participation in all scheduled screens confers the greatest reduction in your risk of dying from breast cancer," said lead author Professor Stephen Duffy.
While the global roll-out of coronavirus vaccines is still in its relative infancy, vaccine-related swelling has been observed in the US.
"We had started to see more patients in our breast imaging clinic with enlarged lymph nodes on mammography, ultrasound and magnetic resonance imaging," said Dr Constance Lehman, from Massachusetts General Hospital.
"We noticed they were coming to our clinic after a recent COVID-19 vaccination."
The medics are keen to encourage coronavirus vaccine uptake, while minimising any unnecessary biopsies or patient anxiety.
"Additional imaging" should not be required for swollen lymph nodes after a coronavirus jab unless the symptom persists for more than six weeks or is accompanied by other health issues, according to the Massachusetts team.
"Enlarged lymph nodes are common after the COVID-19 vaccine and are your body's normal reaction to the vaccine," added the scientists.
Any swelling will likely be more pronounced on the side of the body that had the jab.
Yahoo UK has approached NHS England for a comment.
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