Currently circulating influenza virus is almost universally pandemic 2009 influenza A H1N1, but seasonal influenza strains could also appear soon. Antiviral drugs are an important adjunct to influenza vaccination for treatment and chemoprophylaxis of both pandemic and seasonal influenza. They may, however, interfere with the efficacy of FluMist, the live-attenuated intranasal vaccine, if they are administered within 48 hours before or <2 weeks after FluMist administration. Inactivated vaccines are not affected by antiviral drug therapy.

The FDA has licensed 4 new monovalent vaccines for prevention of respiratory illness caused by a new influenza A H1N1 virus that appears to be derived from a swine strain. The 5th vaccine should be licensed soon. All of these vaccines are expected to become available in October. An intranasal formulation is expected in the first week of October.

Two separate influenza vaccines will be available this season: the usual seasonal trivalent vaccine and a monovalent vaccine (not yet licensed by the FDA) directed at the novel H1N1 virus currently causing pandemic infection. The next issue of the Medical Letter will review the pandemic H1N1 vaccine.

Recent reports (D Kaye.Clin Infect Dis April 1, 2008;46:iii) indicate that 2 of the 3 components of this year's influenza vaccine (Med Lett Drugs Ther 2007;49:81), the influenza B and A/H3N2 antigens, do not matchup well with circulating strains. The B component is probably not protective. The A/H3N2 antigen in the vaccine probably provides some protection, particularly for high-risk patients. The third component, the A/H1N1 antigen, is protective against circulating A/H1N1 influenza viruses, but these viruses, unlike previous A/H1N1 strains, are developing some resistance to oseltamivir (Tamiflu), particularly in Europe; in the US and Canada, about 5% of influenza A/H1N1 isolates are resistant to oseltamivir in vitro.

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Annual immunization against influenza A and B is the most effective method of preventing infection and has been shown to reduce associated complications.

Since the publication of our article on influenza vaccine (October 9, 2006), a fourth inactivated vaccine (FluLaval - GlaxoSmithKline) has been approved by the FDA for use in people ≥18 years old. It is sold in 5-mL multi-dose vials. Each 0.5-mL intramuscular dose contains 25 mcg of mercury.

Annual immunization against influenza A and B is the most effective method of preventing infection and has been shown to reduce associated complications.

Although immunization programs have produced high vaccination rates in US infants and children, similar successes have not been achieved in adults. Vaccines recommended for routine use in adults are reviewed here. Immunizations for travel were reviewed in a recent issue.

Our article on Antiviral Drugs for Prophylaxis and Treatment of Influenza (Med Lett Drugs Ther 2005;47:93) mentioned possible use of amantadine (Symmetrel, and others) and rimantadine (Flumadine, and others), but warned that the incidence of resistance has increased substantially. The CDC recently issued a Health Alert (www.cdc.gov) saying that current evidence indicates that a high proportion of circulating influenza A viruses in the US are now resistant to amantadine and rimantadine and they should not be used for this indication, at least this year. Click here to go to this update article.

Antiviral prophylaxis is indicated for influenza exposures that occur before (or less than 2 weeks after) vaccination with inactivated vaccine, or in years when circulating strains differ from those included in the vaccine. Antiviral drugs can also be used for treatment of patients who develop symptoms of influenza, regardless of vaccination status.
Please see Update: Influenza Resistance to Amantadine and Rimantadine

Annual immunization against influenza A and B is the most effective method of preventing infection and has been shown to reduce associated complications. A future issue of The Medical Letter will review drug prophylaxis and treatment of influenza.

Due to the unanticipated shortage in the US supply of inactivated influenza vaccine, many persons who normally would have received the vaccine will be unable to get it this year. Antiviral drugs can be used for prophylaxis of unvaccinated persons who are exposed to influenza, and for treatment of both vaccinated and unvaccinated patients who develop symptoms of the disease.

Last year influenza season began in October, peaked in December (both much earlier than usual), and was associated with severe disease, particularly in children. Much of the illness was due to a circulating influenza A H3N2 strain (Fujian type) that had drifted antigenically from the H3N2 strain in the vaccine. The vaccine for the 2004-05 influenza season includes A/Wyoming/3/2003 (H3N2), which is antigenically equivalent to the Fujian strain, A/New Caledonia/20/99 (H1N1), which is unchanged, and a new B strain, either B/Jiangsu/10/2003 or B/Jilin/20/2003, which are both antigenically equivalent to B/Shanghai/361/2002.

Unlike the last few years there is no early-season shortage of influenza vaccine. Immunization programs for all recommended individuals can proceed. There are 2 new influenza vaccine formulations available this year, FluMist, an intranasal vaccine and Fluzone, a pediatric formulation. Timing, indications, adverse effects, dosage and cost of the vaccines is discussed. The drugs that can be used for prophylaxis of influenza are also reviewed.

FluMist (MedImmune), the first live-attenuated and first intranasally administered influenza vaccine, has been approved by the FDA to prevent influenza in healthy people 5-49 years old. Given as a nasal spray, it stimulates immunity by viral replication in the upper respiratory tract. This vaccine apparently will be advertised directly to the public as a "needle-free" alternative to intramuscular influenza vaccine.