Influenza can be highly contagious, particularly among persons without pre-existing antibodies against influenza, such as young children during the inter-pandemic phase influenza and anyone during a pandemic. Roughly 50% of all infections are however asymptomatic; asymptomatic infection is especially common in children. The influenza virus is transmitted in most cases by droplets through the coughing and sneezing of infected persons, but it can be transmitted as well by direct contact.
Direct-contact transmission involves skin-to-skin contact and physical transfer of microorganisms to a susceptible host from an infected or colonized person, such as occurs when personnel turn patients, bathe patients, or perform other patient-care activities that require physical contact. Direct-contact transmission also can occur between two patients (e.g., by hand contact), with one serving as the source of infectious microorganisms and the other as a susceptible host. Indirect-contact transmission involves contact of a susceptible host with a contaminated intermediate object, usually inanimate, in the patient's environment. Contact transmission of influenza may occur through either direct skin-to-skin contact or through indirect contact with virus in the environment. Transmission via contaminated hands and fomites has been suggested as a contributing factor in some studies. However, there is insufficient data to determine the proportion of influenza transmission that is attributable to direct or indirect contact.
Droplet transmission occurs when contagious droplets produced by the infected host are propelled a short distance through coughing or sneezing and can come into contact with another person's conjunctiva, mouth or nasal mucosa. Influenza can be transmitted by large droplets, which generally travel 3 to 6 feet. Since these droplets generally are large (greater than 10 micrometers) and do not stay suspended in the air, this mode of transmission is not affected by special air handling or control of room pressures. Large droplets appear to be the primary route of nosocomial [hospital acquired] transmission.
Droplet nuclei (airborne) transmission entails the production of infectious droplet nuclei, generally 5 micrometers or less in diameter. In contrast with larger droplets, these droplets can remain suspended in the air and be disseminated by air currents in a room or through a facility to be inhaled by a susceptible host. Small droplet nuclei and aerosols can remain suspended in the air for prolonged periods and travel significant distances. Small particles appear to be more infectious, with both the degree of infectivity and the severity of illness and is directly related to particle size. Aerosols smaller than 10 microns have been shown to cause more severe disease and require a smaller inoculum than large intranasal droplets. Preventing the spread of droplet nuclei requires the use of special air handling and ventilation procedures.
There is no evidence that influenza transmission can occur across long distances (e.g., through ventilation systems) or through prolonged residence in air, as seen with airborne diseases such as tuberculosis. Organisms transmitted in this manner must be capable of sustaining infectivity, despite desiccation and environmental variation that generally limit survival in the airborne state. However, transmission may occur at shorter distances through inhalation of small-particle aerosols (droplet nuclei), particularly in shared air spaces with poor air circulation. An experimental study involving human volunteers found that illness could be induced with substantially lower virus titers when influenza virus was administered as a small droplet aerosol rather than as nasal droplets, suggesting that infection is most efficiently induced when virus is deposited in the lower rather than the upper respiratory tract.
Direct transmission involves direct body-to-body surface contact. Indirect transmission occurs via contact with contaminated intermediate objects such as contaminated hands or inanimate objects such as needles or countertops. Aside from being actually coughed or sneezed upon by an infected person, the most common way to catch the flu is by touching something which has been coughed on or sneezed upon by an infected person. For instance, the person that used the shopping cart before you had the flu. They covered their mouth with their hand when they coughed then used that very hand to push the cart around the store. Now your hands are touching the same place. Without thinking while shopping, you rub your eye or nose and you have introduce the virus to your most vulnerable point of infection. Good hand washing does more to prevent the spread of flu than anything else.
Evidence supporting the relative contribution of each route of transmission for influenza is limited; however, droplet transmission is thought to be the predominant form of spread in a setting with an appropriate number of air exchanges and standard ventilation. In the absence of appropriate ventilation and air exchange, airborne transmission may play a greater role, such as in a crowded space where air exchange is limited.
Flu Transmissibility / Reproductive Number
Influenza viruses are genetically variable, and transmissibility is difficult to predict. With a novel flu virus the R0 will start out low, probably a little above 1, and then with each generation of transmission it will increase as the virus adapts to the human population. The speed with which transmissibility can improve highlights the unpredictability of influenza viruses. The reproduction, or transmissibility (RO) rates refer to the average number of secondary cases of disease generated by a typical primary case in a susceptible population; an RO rate of 1.0 would thus indicate no transmission.
The reproductive number at a given time, represented as R(t), is the average number of secondary cases infected by each primary case infected at time t. This number must be held steadily below one for the spread of the virus to decline; while this objective may or may not be possible for pandemic influenza without a vaccine, the level of R(t) is perhaps the best single measure of the effectiveness of control measures at a given time.
The reproduction, or transmissibility (RO) rates are situation specific and can be highly variable, with person-to-person transmission probabilities are highest in households; lower in the day-care centers, playgroups, and schools; and even lower in the neighborhoods and population at large. For influenza a person with flu-like symptoms at a workplace may not self-isolate before the end of the working day, which will be a substantial delay on influenza's rapid time scale of development and spread.
Flu may spread rapidly because it has a very short generation time, even if it has a low R0. One study assumed human viral reproduction, or transmissibility rate (RO) [the "reproductive number"], ranging from about 1.0 to 2.0, and set the generation time (Tg), meaning the average interval between infection of an individual and infection of contacts, at 2.6 days. This Tg factor was arrived at on the basis of analysis of past estimates of transmissibility of respiratory diseases and is less than the approximately 4 days assumed in most past modeling studies, say the authors. A predicted attack rate of 50% to 60% derived from these factors was consistent with the first two waves of past flu pandemics.
Influenza, which has a very short generation time, will spread very quickly even if each individual does not spread it to many others. ß(t) can be estimated from experimental infections. Some suggest a mean of 3 days (when variance = 0.5 × mean2), whereas viral shedding peaking at 2 days suggests that S(t) has an estimated mean of 2 days. This results in a range of attack rate estimates of 30% < ? < 50%.
Some analyses report that influenza typically has a transmission rate of about 10. But others suggest that flu is not as highly transmissible in a community setting as has been imagined. The R0 for the 1918 pandemic was estimated to be only 1.8 in one study, while the 1918 pandemic strain's R0 was estimated at around 2 by another estimate. According to another analysis, the estimated proportion of the population with A/H1N1 immunity before September 1918 implied a median basic reproductive number of less than 4. Another study estimated R0=1.89 from influenza case incidence data for the first wave of pandemic influenza A (H3N2) starting in July 1968 in Hong Kong. These results suggested that the reproductive number for 1918 pandemic influenza was not large relative to many other infectious diseases. Other recent estimates of R0 for seasonal and pandemic flu typically range from 1.5 to 3. Estimates of the reproductive number (R) from England and Wales (1958-1973), for a mixture of influenza types and subtypes, ranged from 1.4 to 2.6. In contrast, SARS had an R0 of 3 (excluding super-spreaders), and measles has an R0 of 10 to 15, pertussis (16 - 18) or polio (8 - 12).
Estimates of R0 based on the initial epidemic growth rate may underestimate the true value of R0. Data from an influenza outbreak in an English boarding school has been used to estimate model parameters by trajectory matching. The most commonly used framework for epidemiological systems, the SIR (susceptible - infectious - recovered) model, yields an R0 of 4.38, whereas for the SEIR (susceptible - exposed - infectious - recovered) model yields an R0 of 16.9. A maximum bound for R0 can be obtained by analyzing the case data from an outbreak of the 1978 H1N1 flu in a boys boarding school, yielding an upper bound of R0 < 21.
Another estimate of the reproductive number for 1918 influenza was made by fitting a deterministic SEIR (susceptible - exposed - infectious - recovered) model to pneumonia and influenza death epidemic curves from 45 US cities: the median value was less than three. The estimated proportion of the population with A/H1N1 immunity before September 1918 implies a median basic reproductive number of less than four. These results suggested that the reproductive number for 1918 pandemic influenza is not large relative to many other infectious diseases.
If the basic reproductive number (R0) was below 1.60, some simulations show that a prepared response with targeted antivirals would have a high probability of containing the disease. The higher the R0, however, the lower the likelihood of containing the virus. When the R0 is set at 2.4, for example, the outbreak quickly grows uncontrollably large in most cases of some simulations.
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