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This application is a service of the Singapore Government.

Health Sciences Authority

Summary on Safety Monitoring of H1N1 Vaccines from 3 November 2009 to 31 July 2010

In the interpretation of the AE reports/data, it is important to note the following:

  • The AEs that are reported to HSA are those that the healthcare professionals suspect to be associated with the H1N1 vaccines. These suspicions are usually made from observing a temporal (time-effect) relationship between the occurrence of the event or arrived at after the clinical exclusion of other possible causes of the event. As there are no diagnostic tests for diagnosing an adverse event in general, a suspected AE does not mean that it is confirmed that the vaccine has caused the event. Further observations and studies are needed to confirm the causal link. 

  • A suspected AE report to the H1N1 vaccine may mean that the vaccine could have caused the adverse event, or it could be a coincident event that was associated in time with vaccine administration, but not caused by the vaccine. Confounding factors that may contribute to the latter include previous medical histories of allergies, natural progression of underlying disease conditions (e.g. asthma, high blood pressure, diabetes, other chronic heart conditions), autoimmune disorders and other medicines taken at about the same time as the vaccine or given earlier within a 3 month period. 

  • Suspected AE reporting rates are highly variable and are dependent on many factors. Therefore the data cannot be used to determine the frequency of the occurrence of the adverse reactions to H1N1 vaccines. 

  • The data provided should not be used to draw comparisons on the safety of different brands of vaccines as this is confounded by factors such as extent of use and the patient populations exposed.

In Singapore, the H1N1 vaccines approved are Panvax® (CSL, Australia) and Pandemrix® (GSK). An estimated 440,000 doses of Panvax® and less than 100 doses of Pandemrix®have been distributed to date.


Since the start of the H1N1 vaccination on 3 November 2009, the Vigilance Branch, HSA, has received 171 adverse event reports from healthcare professionals. Majority of the adverse reaction reports were non-serious and described already known adverse reactions that are associated with seasonal flu vaccines. The common side-effects are shown in the table below:

Common Adverse Reactions Percentage of reports (%)
General disorders and administration site conditions (e.g. fever, body ache, flu-like symptoms, fatigue, injection site swelling or pain) 21.5
Skin reactions (e.g. rash, urticaria, itch, periorbital swelling) 21.5
Respiratory system disorders (e.g. sore throat, coughing, rhinorrhoea, nose congestion, asthma, upper respiratory tract infections) 20.6
Nervous system disorders (e.g. headache, giddiness, lethargy, localized numbness) 13.7
Musculoskeletal system disorders (e.g. myalgia, neck/shoulder pain ) 6.5
Gastrointestinal disorders (e.g. nausea, diarrhoea) 5.4

22 reports were assessed as serious* by the reporting physicians. After careful review by HSA and its panel of experts, it was found that in many cases, it was unlikely that the H1N1 vaccines directly caused these adverse events. They could have been precipitated by other causes such as the patient's underlying medical conditions, other medicines that the patient was taking or coincidental events that could have occurred around the time of vaccination.

These reports include seven reports of allergic reactions of varying severity: a case of anaphylaxis, rash and wheezing accompanied by swelling around the eyes, exacerbation of pre-existing scleritis (inflammation of the sclera of the eye), Sweet's syndrome (hypersensitive reaction involving skin and joints), puffy eyes, facial flushing and Churg Strauss syndrome (inflammation of the blood vessels causing sensory disturbances). Six of these patients had pre-existing medical conditions such as asthma or were known to be allergic to certain medications.

Eight other cases include a case of new onset diabetes, a case of persistent vomiting, a case of upper respiratory tract infection with diarrhoea, a case of dystonia (involuntary jerking of the limbs), two cases of facial palsy (weakness of facial muscles) and two cases of Guillain-Barre syndrome (sudden onset of muscle weakness) in patients who had either a recent infection and/or received other concomitant vaccine(s) administration.

There were three cases of H1N1 infection in patients who received vaccination more than 2 weeks prior to the onset of influenza illness. It is to be noted that no vaccine or drug is 100% effective and vaccination failure may still occur in certain instances.

Four other cases were reported in children and these include a case of high fever, a case of hypotonic-hyporesponsive episode (decrease in sensory awareness that is accompanied by pallor and muscle weakness) and two cases of seizures.

HSA's assessment

The adverse events (AEs) received to date for Panvax® vaccine appear similar to the safety profile of seasonal flu vaccines. No unexpected new safety issues have been identified in our local data to date. This is consistent with WHO's finding that the safety profile of the pandemic vaccine is similar to what has been seen in seasonal flu vaccines, based on the over 85 million doses of vaccines that have been administered worldwide.


No suspected AEs have been reported in association with Pandemrix®.

Ongoing Safety Monitoring

HSA is working closely with its international regulatory partners and the World Health Organization to detect and respond to any safety signals that may emerge as the H1N1 influenza vaccine is used around the world.

*The term serious is defined as an event that is assessed to be medical important, or requiring (or prolonging) a hospital admission, or a persistent or significant disability, or a congenital birth defect, or life threatening or fatal.