A recent report in the journal The Lancet Respiratory Medicine provides some surprising information about who might spread influenza. Dr. Andrew C. Hayward of University College, London and colleagues examined five years worth (2006-20011) of data from England of various aspects of influenza infections. They assessed the risk of infection, the severity of illness if infected, rate of illness, and rate of consultation with healthcare providers. Information was derived from caregivers data, blood tests before and after each flu season as well as from nasal swabs of individuals reporting illness.
On average, the researchers determined that 18 percent of unvaccinated individuals were infected with the flu each winter which represents at least 20 million Americans, by the way. Surprisingly, 77 percent of those people showed no symptoms. Further, only about 17 percent of those who were confirmed by tests to be infected actually consulted with their doctor. And of those people who actually had flu- like symptoms such as fever, headache, muscle aches, cough or sore throat only 21 percent consulted their doctors.
What shall we take from such data? In view of the undoubtedly high rates of subclinical influenza infection, an important unanswered question is the extent to which mild and asymptomatic influenza infections contribute to transmission, wrote Dr. Peter William Horby from the Oxford University Clinical Research Unit in Viet Nam. He continued A large number of well individuals mixing widely in the community might, even if only mildly infectious, make a substantial contribution to onward transmission. This might have important implications for the effectiveness of case isolation and social distancing measures in reducing overall transmission rates.
These data are quite disturbing in terms of keeping the spread of influenza in check, said ACSH s Dr. Gilbert Ross. In addition, he continued, the relatively high incidence of asymptomatic infection indicates that our estimates of the extent of influenza infections among the general population is substantially lower than the true rate of infection. If these trends are also operative in the United States, then it provides an additional imperative for healthcare workers to be vaccinated against influenza, as we have long advocated.