The swine flu cycle
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Swine flu has claimed 145 lives this year, most of the deaths taking place in Rajasthan, Punjab, Haryana and Gujarat, according to health ministry figures updated till February 10. Virologists rule out mutation of the H1N1 virus and say the spurt of cases, which follows a lull in 2011, is part of a pattern.
"The virus is cyclical in nature," said Dr M S Chadda, deputy director at National Institute of Virology (NIV) in Pune. Under the pattern, the virus transmits itself rapidly for one or two years and goes into a lull the following year. The pandemic flu had claimed 981 lives in 2009 and 1,763 in 2010, then the toll dropped to 75 in 2011, before rising again to 405 in 2012.
Data put up on the health ministry website cite a total 3,362 such deaths between 2009 and February 10 this year (the year-by-year figures on the same site add up to 3,369).
Maharashtra suffered greatly in 2009, 2010 and 2012 but had a relatively quiet 2011; this year, it has been hit less far severely than some other states have. Gujarat, another major sufferer in the past, has been struck again after a lull in 2011. Other states hit heavily in the past include Kerala, Tamil Nadu and Karnataka, the last of which has had 31 cases this year but reported no deaths.
NIV experts studying the sudden spurt attribute the apparent shift from the south to various factors, including the persistent cold in the northern states. Dr A C Mishra, scientist emeritus at NIV and former director, said one reason why some southern states have not been badly affected is the constant presence of the virus there for the last four years; a large section of the population has developed herd immunity.
Current NIV director D T Mourya and deputy director Chadda said NIV is assessing data from ten surveillance centres. "According to our reports from AIIMS, there was hardly any influenza activity till December last year. However in the past few weeks, there has been an increase in the transmission of the virus," Dr Chadda said.