Review Article Pandemic (H1N1) 2009: Epidemiological, clinical and prevention aspects JAI P. NARAIN, RAJESH KUMAR, RAJESH BHATIA World Health Organization, Regional Office for South-East Asia, New Delhi 110002, India JAI P. NARAIN, RAJESH KUMAR, RAJESH BHATIA Department of Communicable Diseases Correspondence to JAI P. NARAIN; narainj@searo.who.int © The National Medical Journal of India 2009 ABSTRACT The influenza pandemic caused by the new H1N1 virus has by now affected all the continents of the world. However, the extent and likely impact are still uncertain. Like seasonal flu, the illness is mild and self-limiting in a great majority of cases, with only 1%–2% of patients requiring hospitalization. In a few cases, the clinical course can deteriorate in a matter of hours, leading to severe complications and eventually death. The risk of complications is higher among those who have pre- existing diseases, such as asthma, heart disease and kidney disease, and among pregnant women. In such cases, antiviral treatment should not be delayed pending laboratory confirmation. The preferred antiviral drug is oseltamivir, and zanamivir is an alternative. Antiviral treatment is not necessary for those who are otherwise healthy, and have mild or uncomplicated illness. It is beneficial for patients with progressive lower respiratory tract disease or pneumonia, and those with underlying medical conditions and pregnant patients. As the supply of antivirals is limited, they should be used judiciously and where appropriate. There is a limited supply of pandemic influenza vaccine available in a few countries and efforts to produce it in India are presently underway. Effective personal preventive measures include shielding one’s mouth and nose while coughing and sneezing, frequent washing of hands with soap, avoiding mass gatherings and voluntary isolation by symptomatic individuals. While at present the virus is causing a mild disease, the next wave may be more severe. Hence, enhanced surge capacity of health services is required for the clinical management of an increased patient load. Natl Med J India 2009;22:e1–e6 INTRODUCTION Influenza is an old disease caused by influenza virus strains A, B or C. Of these, A is the predominant strain that causes human disease. Influenza viruses are by nature unstable and unpredictable, and have the unique capability of changing their antigenic characteristics by mutation. In the winter and autumn seasons, they cause frequent outbreaks of acute febrile respiratory illness, usually referred to as seasonal flu. Occasionally, novel strains also emerge, often through re-assortment or exchange of genetic material among influenza viruses from different animal, human or bird sources. Such genetic restructuring occurs regularly in nature and, at times, provides the virus with the capability of causing widespread disease in immunologically naïve populations. The virus can move swiftly across geographical borders to cause pandemics. Three such pandemics occurred in the previous century, in 1918, 1957 and 1968. The 1918 pandemic was the most devastating, taking a toll of 30–40 million lives worldwide. The subsequent pandemics were relatively milder, each killing around 1 million people. 1 The year 2003 witnessed the appearance of a novel avian influenza A subtype (H5N1), which caused 438 cases and 262 deaths in 15 countries. It remains endemic in poultry populations in many countries, including some countries of the Southeast Asia region, in particular, Indonesia, and occasionally leads to the occurrence of human cases. 2 In March 2009, another novel strain of influenza virus A (H1N1), resulting from triple re-assortment, emerged in Mexico and the USA. 3,4 In late April 2009, the WHO declared that the emergence of this virus represented a ‘public health emergency of international concern’ and on 11 June 2009, raised the phase of pandemic alert to 6, indicating that a new influenza pandemic was under way. 5 The H1N1 virus has spread with great speed and ease to all continents and is causing considerable human suffering. It is also having an adverse impact on the health services and the economy. THE NOVEL VIRUS AND ITS CHARACTERISTICS The 2009 H1N1 virus contains a combination of gene segments that have previously not been reported in swine or human influenza viruses. 6 Its genome is the result of a re-assortment of genes from 4 influenza viruses, i.e. North American swine influenza, Asia/ Europe swine influenza, human influenza and avian influenza (non-H5) (Table I). 7 The viruses isolated from humans during the course of this epidemic have been homogeneous, with a difference of a maximum of only 5 amino acids among them. On the basis of current knowledge, there is no known molecular evidence of genetic changes to this virus which would explain its transmissibility among people. Some of the differences between THE NATIONAL MEDICAL JOURNAL OF INDIA VOL. 22, NO. 5, 2009 e1