The uncleaved viral hemagglutinin HA0 increases influenza A virus resistance to thermal pasteurization.

Two biological types of influenza virus are known and distinguished by the structure of the surface glycoprotein, hemagglutinin (HA). The noninfectious virions contain the uncleaved HA0 (80 kDa), whereas the infectious type have a cleaved form of two subunits, HA1 (55 kDa) and HA2 (25 kDa). The point cleavage of HA0→HA1+HA2 by host proteases regulates the virion integrity to maintain the functionality of the intravirion axis HA→M2→M1→RNP. The HA0 containing virions are more resistant than the HA1/HA2 virions to the 75 °C temperature used in pasteurizing milk and foods. The noninfectious HA0 virions treated at 75 °C were able to retain infectious potential, which was activated by trypsin; in contrast, the infectious HA1/HA2 virions lost infectivity irreversibly under pasteurization. The data suggest that (i) influenza viruses retain their infectious potential in the external environment by means of noninfectious virions containing the uncleaved HA0 and (ii) a stronger pasteurization regimen in terms of temperature and duration of thermal treatment is recommended to inactivate such potentially infectious virions in food products.