by Charles John Palenik, MS, PhD, MBA
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Influenza is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness and, at times, can even lead to death. In the United States, annual epidemics of influenza typically occur late fall through early spring. It is common to refer to these yearly winter epidemics as seasonal or interpandemic influenza. Sporadic, unpredictable pandemics can occur. There were three influenza pandemics in the 20th century.
Influenza viruses infect between 5% and 20% of the U.S. population annually. Approximately 36,000 influenza-associated deaths occur annually in the United States. More than 90% of these deaths involve people 65 years and older. Higher mortality occurs in seasons when influenza type A (H3N2) viruses predominate.
On average, there are 200,000 influenza-related hospitalizations annually. The highest rates of complications and hospitalization are among young persons and those 65 and older. Greater numbers of hospitalizations occur during type A (H3N2) epidemics. The economic impact of influenza is substantial, averaging $87.1 billion annually.
Annual vaccination is the most effective method for preventing influenza virus infection and its complications. The vaccine can be administered to any person six months or older, who does not have contraindications to vaccination, to reduce the likelihood of becoming ill with influenza or of transmitting influenza to others.
The effectiveness of the influenza vaccine depends, in part, on the match between the vaccine and influenza viruses that are circulating in the community. If these matches are close, vaccine effectiveness (VE) is high with values as great as 70% to 90% for inactivated vaccines. If the match is not close, VE is reduced.
A study in Wisconsin found an overall VE of 44% for the 2007-08 influenza vaccine. This included a 58% VE against the predominant influenza A (H3N2) strain and no effectiveness against influenza B viruses. No influenza A (H1N1) viruses were present in the population studied.
A higher VE against H3N2 viruses than against influenza B viruses was likely because the circulating H3N2 strains and the H3N2 strain in the vaccine were closely related while the circulating B viruses were antigenetically and genetically much different from the influenza B virus in the vaccine. The result was little to no cross-reactivity.
The current study and others indicate that vaccination can provide some cross-protection against different but related viruses. Different influenza viruses can circulate in different parts of the United States at various times. In order to be scientifically valid, there must be a representative sampling taken nationally.
In the United States, the influenza vaccine has a good record of accomplishment, being well-matched 16 of the last 19 influenza seasons. The last problem occurred in 2003-04, when Fujian influenza emerged in Asia too late to be included in the vaccine formula.
Influenza viruses are constantly changing. They can change from the time the influenza vaccine recommendations appear to the beginning of the influenza season. Each year, experts study thousands of flu virus samples from around the world to determine which viruses are making people sick and how these viruses are changing. With this information, agencies forecast the three viruses that are most likely to affect the population during the next flu season.
Each seasonal influenza vaccine contains three influenza virus strains — one influenza A (H3N2) virus, one influenza A (H1N1) virus, and one influenza B virus. The selection of which viruses to include in the vaccine must be made the February before the start of a new influenza season. Vaccine production requires a prolonged ramp-up period because it currently involves incubation of influenza viruses in chicken eggs.
Vaccine composition is part science and part intuition. For this reason, there is always the possibility of a less than optimal match between viruses in the vaccine and circulating viruses.
The influenza A Brisbane 10 strain was the problem last year. It emerged too late to be part of the 2007-08 vaccine and caused disease later in the season, especially during February. For the 2008-09 season, the FDA — for the first time — replaced all three types of influenza viruses in the vaccine.
The 2008–09 trivalent vaccine virus strains are A/Brisbane/59/2007 (H1N1)-like, A/Brisbane/10/2007 (H3N2)-like, and B/Florida/4/2006-like antigens.
Dr. Charles John Palenik is the director of Infection Control Research and Services at the Indiana University School of Dentistry. In 2003, he was chairman of the Executive Board of OSAP, dentistry's resource for infection control and safety. Direct infection control questions to [email protected].
