Influenza A (H1N1-2009) outbreak

by Pek, Sara

H1N1 was first detected in the United States in April 2009. This virus was a unique combination of influenza virus genes never previously identified in either animals or people. The virus genes were a combination of genes most closely related to North American swine-lineage H1N1 and Eurasian lineage swine-origin H1N1 influenza viruses. Because of this, initial reports referred to the virus as a swine origin influenza virus. However, investigations of initial human cases did not identify exposures to pigs and quickly it became apparent that this new virus was circulating among humans and not among U.S. pig herds.1

The symptoms of H1N1 are similar to those of seasonal influenza and include fever, sore throat, cough, body aches, runny nose, headaches and tiredness.2 Unlike seasonal influenza, the H1N1 virus tends to strike a higher proportion of young adults and those with underlying medical conditions.3 Most cases of H1N1 infection in Singapore were mild, although up to 18 deaths due to H1N1 were reported in 2009.4

Global outbreak in 2009
The United States Centres for Disease Control and Prevention (CDC) first isolated two Southern Californian children who had fallen ill from a new strain of virus in late March 2009, and posted an early alert on its Morbidity and Mortality Weekly Report website. The posting alerted the Mexican authorities, as there was a surge in influenza-like illnesses in March and early April in several areas of Mexico. The alarming situation pointed to the possibility that the cases were due to the influenza A (H1N1) virus.5

The World Health Organization (WHO) declared the H1N1 infection a public health emergency on 24 April.6 This was followed by the establishment of an extensive network of international monitoring systems in preparation for the outbreak.7

Concurrently, the Mexico government imposed a shutdown of public venues in its metropolis, where the outbreak had sickened the most number of people, to curb the flu's spread.8 The illness soon spread beyond Mexico, the epicentre of infections, to North America, Europe and Asia.9

On 30 April, the WHO Phase of pandemic alert was raised from Phase 4 to Phase 5, which is characterised by human-to-human spread of the virus in at least two countries in one WHO region.10 The outbreak continued to spread, with an increasing number of confirmed H1N1 cases due to a backlog of cases awaiting confirmation. Countries, territories and areas with confirmed cases was made available on the WHO website.11

H1N1 in Singapore

Prior to the H1N1 outbreak in Singapore, there already existed a disease surveillance system and influenza pandemic preparedness plan. Known as the Disease Outbreak Response System (DORS), it is a five-colour alert system that progresses from green to yellow, orange, red and black.12

In response to the H1N1 outbreaks, the Singapore Ministry of Health (MOH) took precautionary health measures to contain the spread of the virus to Singapore. These health control measures included contact tracing, the issuing of Home Quarantine Orders (HQO), travel advisories, public health advisories (temperature screening at schools, public and commercial places including border checkpoints and screening for flu-like symptoms for visitors to hospitals), treatment and vaccination.13 Singaporeans were also reminded to ensure a high standard of personal hygiene.14

On 28 April 2009, MOH raised the alert mode from green to yellow (an indication of the situation when the virus evolves into a human disease) for the first time, and then two days later to orange. MOH then revised it downwards to yellow on 11 May. At that time, there were no confirmed cases of infection in Singapore.15

Singapore confirmed its first case of H1N1 in late May 2009. The patient, a 22-year-old Singapore Management University student, had travelled from New York back to Singapore on 26 May and had developed a cough while on the flight. She passed the airport thermal scanner without incident as she did not have a fever at the time. Later that morning, she consulted a doctor who sent her to Tan Tock Seng Hospital via a 993 ambulance, given her travel history. She was immediately admitted for testing, after which laboratory confirmation of her infection was made by midnight of 26 May. MOH initiated contact tracing of her close contacts, who were quarantined, and provided with antiviral prophylaxis.16

By 24 June, there were 220 human cases of H1N1 in Singapore.17 To ensure that Singaporeans had ready access to medical assessment and prompt treatment for mild cases of H1N1, more than 400 family clinics island-wide were made H1N1-ready.18 Supplies of Personal Protection Equipment and the Tamiflu antiviral drug were made available at these Pandemic Preparedness Clinics.19

On 18 July, a 49-year-old man with heart problems became the first person to die after being infected with the H1N1 virus.20 The victim, who had multiple health problems, died of a heart attack complicated by severe pneumonia and the H1N1 infection. The virus outbreak appeared to peak by August, with attendances at clinics dropping off for the first time in weeks. The 18th and last H1N1 fatality of the year, in September, was a 61-year-old man with a history of diabetes, hypertension and end-stage renal failure.21

Singapore received its first batch of the H1N1 vaccine in end-October 2009. Further checks and clearance were completed in less than a week, and mass vaccinations began on 3 November 2009. Then Health Minister Khaw Boon Wan assured that the supply of the H1N1 vaccine "should be enough". By the end of 2009, one million does were available.22

On 12 February 2010, the Health Ministry changed its alert level to green.23 By this time, it was estimated that 415,000 people in Singapore had been infected with the H1N1 virus and more than 420,000 local residents had received the H1N1 vaccine injection.24 Most infected persons in Singapore experienced mild illness, while there were more than 1,600 hospital admissions from complications related to the virus, including about 100 admissions to intensive care.25 Two weeks after the alert status turned Green, a 5-year old boy with no underlying medical conditions died from the virus infection at the KK Women's and Children's Hospital.26

The Singapore Communicable Disease Centre (CDC) conducted a study on the H1N1 virus from June to October 2009, before, during and after the H1N1 epidemic. Released in April 2010, the report found that most adults in Singapore remain vulnerable to the H1N1 virus as only 13 percent of the population has natural immunity to it. Singaporeans were advised to get vaccinations against both H1N1 as well as the seasonal flu.27

Though mild, the H1N1 virus continues to be the predominant influenza strain in Singapore and globally. Overall, influenza activity in Singapore remains at low to moderate levels. While most people with H1N1 recover with rest and treatment, infection with the virus can result in hospitalisation or even death, especially in those with underlying medical conditions such as asthma, diabetes, gross obesity or low immunity, or in those who are pregnant.28

On 10 August 2010, the WHO declared an end to the 2009 H1N1 flu pandemic. However, it cautioned that the virus would likely return as a seasonal flu, and added that the H1N1 flu vaccines should still be used.29


Sara Pek

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9. Voice of America. (2009, December 25). Swine flu tops list of 2009 health issues. Retrieved 2020, March 30 from Voice of America website:
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11. World Health Organization (2010, August 6). Emergencies preparedness, response. Retrieved 2016, August 3 from World Health Organization website:
12. Ng, W.C. (2009, May 9). EXPOSED. Some clinics let their guard down. The New Paper. Retrieved from NewspaperSG.
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; Hooi, J. (2009, June 12). Singapore preparing for next phase of flu. The Business Times, p. 9. Retrieved from NewspaperSG.
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15. Tay, J., (2010, April). Influenza A (H1N1-2009) Pandemic in Singapore – Public health control measures implemented and lessons learnt. Annals, Academy of Medicine Singapore, p. 313. Retrieved 2016, August 3 from:
16.  Ministry of Health. (2009, May 27). First confirmed case of Influenza A (H1N1-2009) in Singapore. Retrieved 2020, March 30 from Ministry of Health website:
17. Ministry of Health. (2009, June 24). 26 new confirmed cases of Influenza A (H1N1-2009) [Press release]. Retrieved 2020, March 30 from National Archives of Singapore website:
18. Teo, W.G. (2009, June 28). Gradual change in handling flu. The Straits Times, p. 7. Retrieved from NewspaperSG.
19. Salma Khalik. (2009, June 24). 450 private clinics HINI ready. The Straits Times, p. 5; Tan, J., & Ong, C. (2009, May 19). 60 more clinics sign up to tackle H1N1 cases. The Straits Times, p. 24. Retrieved from NewspaperSG.
20. Nur Dianah Suhaimi. (2009, July 19). First H1N1-related death in Singapore. The Straits Times, p. 1. Retrieved from NewspaperSG.
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25. Ministry of Health. (2010, February 12). Reverting to green alert status for Influenza A (H1N1) pandemic. Retrieved 2020, March 30 from Ministry of Health website:
26. Sim, M. (2010, February 26). Boy, 5, is 21st to die from H1N1. The Straits Times, p. 8. Retrieved from NewspaperSG.
27. News-Medical.Net. (2010, April 15). Swine flu shots important before peak season, say experts. Retrieved 2016, August 3 from News-Medical.Net website:
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29. WHO declares an end to H1N1 flu pandemic. (2010, August 11). The Straits Times, p. 17. Retrieved from NewspaperSG.

The information in this article is valid as at March 2020 and correct as far as we are able to ascertain from our sources. It is not intended to be an exhaustive or complete history of the subject. Please contact the Library for further reading materials on the topic.


Health and medicine>>Diseases>>Communicable diseases
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H1N1 influenza--Singapore
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H1N1 Influenza Pandemic, Singapore, 2009