But "these reactions are non-serious anticipated side effects such as fever, rashes, flu-like symptoms, headaches, nausea and vomiting."
A HSA spokesperson said "These reactions are commonly expected from all flu vaccines and most of these reactions are resolved within a few days."
MediaCorp understands that one Mrs Wee - a 37-year old supply chain assistant manager - suffered serious side-effects when she, her husband and maid took the jab at Tan Tock Seng Hospital on 7th November.
She had "insect bite-like" on her left forearm, to red blotches "all over her limbs" within a day.
Her wrists, elbows, knees and ankles also swelled up to twice their sizes, making it painful for her to walk.
Mrs Wee is believed to be the first case of someone suffering from serious side effects after taking the H1N1 vaccine jab, since the doses were made available earlier this month.
Even as the H1N1 vaccine has a "relatively good safety profile", the World Health Organisation told AFP an "unusual number of severe allergic reactions" to the H1N1 vaccine have been reported in Canada.
"The Canadian authorities are conducting the appropriate investigations on the vaccines" and "recalled a batch of vaccine from GlaxoSmithKline," said a spokesperson.
A spokeswoman for the UK drugmaker also told Bloomberg, Canadian doctors should hold off on using one particular batch of GSK's swine flu vaccine temporarily while the company probes reports of higher-than-expected occurrences of a side effect known as anaphylaxis - where one suffers from swollen tongues and throats, leading to breathing difficulties and dizziness.
Typically, it affects one in 100,000.
HSA said it is closely monitoring the safety of the H1N1 vaccines that are available in Singapore, and doctors are "strongly encouraged" to report all cases where patients have suffered serious side effects due to the jabs. - TODAY/vm