Update on 2009 pandemic influenza A (H1N1) virus

2009 PANDEMIC H1N1 IS A MONGREL

There are three types of influenza viruses, designated A, B, and C. Type A undergoes antigenic shift (rapid changes) and antigenic drift (gradual changes) from year to year, and so it is the type associated with pandemics. In contrast, type B undergoes antigenic drift only, and type C is relatively stable.

Influenza virus is subtyped on the basis of surface glycoproteins: 16 hemagglutinins and nine neuraminidases. The circulating subtypes change every year; the current circulating human subtypes are a seasonal subtype of H1N1 that is different than the pandemic H1N1 subtype, and H3N2.

The 2009 pandemic H1N1 is a new virus never seen before in North America.⁶ Genetically, it is a mongrel, coming from three recognized sources (pigs, birds, and humans) which were combined in pigs.⁷ It is similar to subtypes that circulated in the 1920s through the 1940s.

Most influenza in the Western world comes from Asia every fall, and its arrival is probably facilitated by air travel. The spread is usually unidirectional and is unlikely to contribute to long-term viral evolution.⁸ It appears that 2009 H1N1 virus is the predominant strain circulating in the current influenza season in the Southern Hemisphere. Virologic studies indicate that the H1N1 virus strain has remained antigenically stable since it appeared in April 2009. Thus, it appears likely that the strain selected by the United States for vaccine manufacturing will match the currently circulating seasonal and pandemic H1N1 strains.

VACCINATION IS THE FIRST LINE OF DEFENSE

In addition to the trivalent vaccine against seasonal influenza, a monovalent vaccine for pandemic H1N1 virus is being produced. The CDC has indicated that 45 million doses of pandemic influenza vaccine are expected in October 2009, with an average of 20 million doses each week thereafter. It is anticipated that half of these will be in multidose vials, that 20% will be in prefilled syringes for children over 5 years old and for pregnant women, and that 20% will be in the form of live-attenuated influenza vaccine (nasal spray). The inhaled vaccine should not be given to children under 2 years old, to children under 5 years old who have recurrent wheezing, or to anyone with severe asthma. Neither vaccine should be given to people allergic to hen eggs, from which the vaccine is produced.

An ample supply of the seasonal trivalent vaccine should be available. Once the CDC has more information about specific product availability of the pandemic H1N1 vaccine, that vaccine will be distributed. It can be given concurrently with seasonal influenza vaccine.

Several definitions should be kept in mind when discussing vaccination strategies. Supply is the number of vaccine doses available for distribution. Availability is the ability of a person recommended to be vaccinated to do so in a local venue. Prioritization is the recommendation to vaccination venues to selectively use vaccine for certain population groups first. Targeting is the recommendation that immunization programs encourage and promote vaccination for certain population groups.

The Advisory Committee on Immunization Practices and the CDC recommend both seasonal and H1N1 vaccinations for anyone 6 months of age or older who is at risk of becoming ill or of transmitting the viruses to others. Based on a review of epidemiologic data, the recommendation is for targeting the following five groups for H1N1 vaccination: children and young adults aged 6 months through 24 years; pregnant women; health care workers and emergency medical service workers; people ages 25 through 64 years who have certain health conditions (eg, diabetes, heart disease, lung disease); and people who live with or care for children younger than 6 months of age. This represents approximately 159 million people in the United States.

If the estimates for the vaccine supply are met, and if pandemic H1N1 vaccine requires only a single injection, there should be no need for prioritization of vaccine. If the supply of pandemic H1N1 vaccine is inadequate, then those groups who are targeted would also receive the first doses of the pandemic H1N1 vaccine. It should be used only with caution after consideration of potential benefits and risks in people who have had Guillain-Barré syndrome during the previous 6 weeks, in people with altered immunocompetence, or in people with medical conditions predisposing to influenza complications.

A mass vaccination campaign involving two separate flu vaccines can pose challenges in execution and messaging for public health officials and politicians. In 1976, an aggressive vaccination program turned into a disaster, as there was no pandemic and the vaccine was associated with adverse effects such as Guillain-Barré syndrome. The government and the medical profession need to prepare for a vaccine controversy and to communicate and continue to explain the plan to the public. As pointed out in a recent op-ed piece,⁹ we would hope that all expectant women in the fall flu season will get the flu vaccines. We also know that, normally, one in seven pregnancies would be expected to miscarry. The challenge for public health officials and physicians will be to explain to these patients that there may be an association rather than a causal relationship.

In health care workers, the average vaccination rate is only 37%. We should be doing much better. Cleveland Clinic previously increased the rate of vaccination among its employees via a program in which all workers must either be vaccinated or formally declare (on an internal Web site) that they decline to be vaccinated.¹⁰ This season, even more resources are being directed at decreasing the barriers to flu vaccinations for our health care workers with the support from hospital leadership.