Determining Existing Human Population Immunity as Part of Assessing Influenza Pandemic Risk
Jonathan Tin Lai Cheung, Tim K. Tsang, Hui‐Ling Yen, Ranawaka A. P. M. Perera, Chris Ka Pun Mok, Yong Lin, Benjamin J. Cowling, Malik Peiris
Abstract
A n influenza pandemic can occur when an in- fluenza A virus with gene segments derived in part or whole from animal viruses becomes able to efficiently and sustainably transmit among humans (1,2). Lack of prior immunity among the human population to the hemagglutinin (HA) of a novel virus enables pandemic spread of that virus. New influenza vaccines require >7 months to develop, but pandemics spread faster than that; a new vaccine would not be available in time to prevent a first pandemic wave, as was seen during the 2009 influenza (H1N1) pandemic (1,3). Because of this delay, surveillance and risk assessment are used to anticipate pandemic threats (4,5), enabling preemptive vaccine development to be initiated. Prepandemic actions might include developing vaccine seed strains, experimental vaccine seed lots, or even phase 1 clinical trials of prepandemic vaccine candidates, depending on risk assessment data. The World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) developed the Tool for Influenza Pandemic Risk Assessment and Influenza Risk Assessment Tool in response to the need for standardized and transparent tools to assess the pandemic potential of influenza viruses (5,6). Based on the properties of the virus, attributes in the human population, and virus ecology in animal hosts (6), such assessments attempt to determine emergence risk, the potential of an animal virus to become able to efficiently transmit among humans, and effect risk, the effect and severity if that virus were to spread among humans. Population immunity is an important feature of assessing risk.