By: David M. Morgan, et al; National Institute of Allergy and Infectious Diseases
28 September 2010
Abstract: History suggests that the 2009 pandemic H1N1 influenza virus faces extinction unless it mutates to avoid already high global population immunity. The immune escape mechanisms potentially at its disposal include antigenic drift, antigenic shift via genetic reassortment, and intrasubtypic reassortment. Going back to the late 19th century, the evolutionary histories of past pandemic viruses are examined in an effort to better understand the nature and extent of the immune pressures faced by the 2009 pandemic virus in the immediate future. While human influenza viruses have often surprised us, available evidence leads to the hope that the current pandemic virus will continue to cause low or moderate mortality rates if it does not become extinct.
(exerpts) Population immunity against pH1N1 in 2010-2011 will comprise a combination of factors, including the immunity raised by prior exposures to cross-reacting viruses and vaccines, the extent of infection with pH1N1, and the relative proportion of the population vaccinated against the virus in 2009-2010. Potential evasive responses of a virus to population immunity include antigenic drift (sequential antigenic changes in hemagglutinin [HA] that lead to immune escape), antigenic shift (importation by reassortment of a gene encoding a different HA subtype with or without additional viral genes), and intrasubtypic reassortment (ISR; importation by reassortment of a gene encoding an antigenic variant of the same HA subtype with or without other genes [3, 4]).