Do public health and infection control measures prevent the spread of flu?

Not much is known about influenza transmission or ways to prevent it. An effective response to an influenza pandemic will hinge on the availability of vaccine and antivirals. If they are in limited supply, as is anticipated, then disease control will depend on public health and infection control measures. Unfortunately, not much is known about the efficacy of such interventions for preventing the transmission of flu. In fact, very little is known about influenza transmission. A review of the literature found no English-language experimental studies of person-to-person spread of flu and no studies of the use of masks or hand-washing as means of preventing transmission of flu. Despite the absence of scientific data, some reasonable inferences and extrapolations can be drawn based on what is known, which is discussed below. Asymptomatic carriers play an important role. As many as half of all infections with normal seasonal flu may be asymptomatic, which in part may be due to preexisting partial immunity.1 Asymptomatic patients shed virus and can transmit the disease but not at the same rate as symptomatic individuals, thus creating an invisible “reservoir” for the virus. The implication of this is that public health disease containment measures and infection control measures alone may slow but cannot stop a flu epidemic. Isolation of the sick may slow the spread of flu. The degree of viral shedding in flu is directly proportional to the severity of symptoms and level of fever.1 Therefore, virus is shed to a greater degree by symptomatic individuals as compared with those who are asymptomatic. As a result, isolating the sick can be expected to reduce transmission and therefore slow the spread of disease. Flu may not be as transmissible as imagined. The incubation period for seasonal flu averages 2 days, with a range of 1 to 4 days, and viral shedding may begin as much 24 hours before symptoms appear.1 The generation time, which is the time between when the first person becomes infectious and when a person in the next “generation” becomes infectious, is estimated to be approximately 2 days as well. The speed with which a contagious disease spreads is a function of the number of additional people infected by each victim (R0) and the generation time. Influenza, which has a very short generation time, will spread very quickly even if each infected individual does not spread it to many others. The R0 for the 1918 pandemic is estimated to be only 1.8.2 Other recent estimates of R0 for seasonal and pandemic flu typically range from 1.5 to 3.3,4 In contrast, SARS had an R0 of 3 (excluding super-spreaders), and measles has an R0 of 10 to15, 5