І. The current status of influenza vaccines

More misinformation exists about influenza vaccine, among the public and in the medical profession, than about any other immunizing agent in common use. Physicians are confused. Uncertainty about the character and frequency of reactions to vaccine, the continuous parade of apparently 'new' influenza viruses, seemingly contradictory information on the effectiveness of influenza vaccine, the necessity of annual immunization, and the difficulty diagnosing influenza all combine to give influenza vaccine an image that is something less glowing.

Fundamental in an understanding of the problem is an appreciation of the antigenic changes that continuously occur, principally among influenza A viruses, but to a lesser extent among the B viruses. There changes, in pseudotechnical terms, are called 'shifts' and 'drifts'.

Antigenic 'shifts' are major antigenic changes occurring at about ten year intervals, such as the appearance of A-prime influenza in 1947, 'Asian' influenza in 1957, and 'Hong Kong' influenza in 1968. Antigenic 'drift' signifies changes of lesser magnitude that occur almost continuously, resulting in the emergence of so-called variant strains. Immunologically, these resemble, but are not identical, the strains circulating previously. For example, in 1972 the 'England' strain of influenza A was widely prevalent in the United States, this having 'drifted' antigenically from the 'Hong Kong' strain. Thus influenza vaccine containing the 'Hong Kong' influenza A virus provided some protection against the 'England' variant in 1972, but the degree of protection was less than optimal.

An appreciation of this phenomenon illuminates two areas of confusion. First, many of the disparate results of efficacy trials can be explained: the influenza virus causing disease in the population under study was not precisely the same as that containing in the vaccine used (drift).

Second, this continuous antigenic change creates a major problem for the Bureau of Biologists of the Food and Drug Administration, the Center of Disease Control, and the manufacturers of influenza vaccine. In order to market an optimally effective product, one must annually gaze into crystal ball, anticipate the influenza virus that is going to circulate a year hence, and then manufacture vaccine with that strain. This annual guessing game is not always successful, but in the past ten years, remarkably enough, it has worked increasingly well.

International influenza surveillance network. Chief among the factors responsible for this success is the worldwide influenza surveillance network operated by the world Health organization, of which the Center for Disease control of the Public Health Service is the domestic arm. Representative strains of influenza virus from all over the world are continuously examined in order to identify antigenic changes as soon as possible. In general, there is remarkable uniformity of circulating influenza viruses throughout the world, and the appearance of a significant antigenic change in one part of the world is usually a signal that this variant strain will rapidly replace the others.

Under optimal conditions, i.e. when the circulating wild virus is closely related or identical to that in the vaccine, vaccination may reasonably be expected to provide from 80% to 90% protection against influenza. Needless to say, influenza vaccine, cannot be expected to provide any protection whatsoever against other respiratory infections, such as those caused by parainfluenza viruses, adenoviruses, coxsackieviruses, rhinoviruses, and the like. This point would seem self-evident, but it is difficult to determine what is influenza and what is not. There are many flulike syndromes that cannot be distinguished from influenza even by experienced clinicians. Influenza can be diagnosed with certainly only by serologic tests or by isolation of the virus.

Official recommendation. The Public Health Service Advisory Committee on Immunization Practices recommends as follows: Annual vaccination is strongly recommended for persons of all ages who have chronic conditions as 1) heart disease of any etiology, particularly with mitral stenosis or cardiac insufficiency 2) chronic bronchopulmonary diseases such as asthma, chronic bronchitis, bronchiectasis, and emphysema, 3) chronic renal disease and 4) diabetes mellitus and other chronic metabolic disorders.

Annual vaccination is recommended for older persons, particularly those over 65 years, because influenza outbreaks are commonly associated with excess mortality in older ages groups.

Дата добавления: 2015-11-14