Michael B. Gregg

THE EPIDEMIOLOGY OF INFLUENZA IN HUMANS

The epidemiology of influenza infection has been extensively studiedparticularly since 1933, when the virus was first isolated. In this presentation I will summarize briefly what has been observed and concluded to be true about its epidemiology, touch upon a few of the more perplexing aspects of the spread of influenza, and then recount some unusual epidemics, particularly a recent extraordinary influenza outbreak that may shed some light on the still unanswered questions concerning the transmission of influenza.* Although influenza remains one of the major uncontrollable epidemic communicable diseases still affecting the world, our measurements of its extent and impact are imprecise and often indirect. Reporting of influenza cases in the United States is not required because the illness can be difficult to diagnose, and many who are sick do not seek medical care. Virus isolation or serologic test results can often confirm the diagnosis, but neither patient nor physician materially benefits from such information. Therefore, health officials and epidemiologists have relied upon either unique, specialized, community-based studies from which broad conclusions are applied regionwide or countrywide or upon indirect assessments of influenza-related mortality as indicators of the frequency and distribution of influenza A. Despite these limitations, yearly and secular trends of the illness do reflect the overall character and distribution of influenza infection. In general, influenza occurs in three relatively distinct epidemiologic forms: 1. worldwide pandemics, 2. regional or nationwide epidemics, and 3. local, sporadic outbreaks. Worldwide or pandemic influenza appeared in 1889, 191718, 1957-58. and 1968-69, usually coincident with major antigenic changes in the influenza virus. Such epidemics may begin at any time, extend for many months, affect 20% to 40% of the population, and seem to arise from a single focus of infection which often can he followed within countries and from continent to continent. Larger regional or nationwide epidemics have occurred fairly regularly every two to three years during winter months, have affected perhaps 570 to 10% of the population, have usually been associated with minor antigenic changes in the virus, and have appeared nearly simultaneously in communities throughout large regions of the country. In the United States and elsewhere, sharply localized, sporadic outbreaks occur nearly every respiratory season and affect small, separate, and discrete populations. Characteristic of all three epidemic forms of influenza are their short-lived duration in the communitythey usually last two to four weeks or eight to ten weeks for the entire United States during interpandemic times-and the remarkable similarity of disease from epidemic to epidemic.

English Flu — A Primer

Influenza is now widespread in the United States. The influenza virus responsible for this outbreak is a moderately distinctive variant of the Hong Kong virus that was prevalent in this country from 1968 until 1972. The virus was first noted in Southern India (Coonoor) from July to November, 1971. It was also isolated from a patient in England in late January, 1972, and the name of the variant (A/England/42/72 [H3N2]) is based on that isolation. In neither country did it cause widespread disease. The virus was recognized by the World Influenza Center (WIC) in London as being antigenically different from .xa0.xa0.

Influenza research in the Soviet Union--1974.

tamination procedures must be responsible. A means for interruption of such transmission must be sought. (g) Risk figures for patients of HB Agpositive physicians, surgeons, and dentists should be developed. If carriage of HG Ag is hazardous to patients, the risk should be defined, and appropriate measures developed for reduction of the risk. (h) The effect of the season on dissemination of herpesviruses should be studied. In addition, the cytomegalovirus group should be evaluated for antigenic variants, since there may be considerable variation in potential for disease among different members of this group. (i) Measures for the increase of specific resistance to representatives of the herpesvirus group should be evaluated. While the effectiveness of specific attenuated or inactivated vaccines may appear to be unlikely on theoretical grounds, transfer factor and other cellular immunological approaches should be studied. (3) Personnel, physicians, and nurses must be trained in hospital epidemiology, surveillance, and control procedures, to implement and to increase the available knowledge. In addition, innovative approaches to the dissemination of knowledge concerning the usage of antimicrobial agents, hazards of various invasive procedures, and hospital control practices must be encouraged.