Influenza

An outbreak caused by a novel strain of H1N1 influenza, commonly called "swine flu", began in Mexico in March 2009 and spread rapidly to many other countries. The virus contains a unique combination of swine, avian, and human influenza gene segments that had not been previously observed. Initial reports from Mexico indicated a high fatality rate in previously healthy young adults and older children, raising concerns that a worldwide pandemic might occur, similar to 1918. However, subsequent data from Mexico, as well as experience from other countries, indicated the H1N1 strain from 2009 is not nearly as lethal as some people initially feared. Preliminary data indicate that up to one-third of those greater than 60 years of age have antibodies against the novel H1N1 virus, whereas protective antibodies are found much less frequently in children and younger adults (see MMWR). This is consistent with the clinical experience, which indicates that the most severe cases have occurred in those younger than age 60.

On August 10, 2010, the World Health Organization declared the H1N1 influenza pandemic was over, saying worldwide flu activity had returned to typical seasonal patterns and many people had immunity to the virus.

The World Health Organization does not recommend any travel restrictions at this time. To protect yourself from H1N1 influenza, wash your hands regularly and avoid close contact with anyone who is coughing or sneezing. Routine use of face masks is not recommended. The symptoms of H1N1 influenza include fever, cough, sore throat, body aches, headache, chills and fatigue, similar to seasonal influenza. Any traveler who develops flu-like symptoms after travel to a country which has reported H1N1 influenza should immediately seek medical attention. Empiric treatment with oseltamivir (Tamiflu) and zanamivir (Relenza) should be considered for suspected cases. A small number of Tamiflu-resistant isolates have been described, but these strains have retained sensitivity to Relenza. The virus is uniformly resistant to amantadine and rimantadine. Vaccination against novel H1N1 influenza is recommended for everyone, except those allergic to the vaccine or one of its components. For further information on novel H1N1 influenza (swine flu), go to the World Health Organization and the Centers for Disease Control.

As of May 2011, worldwide influenza activity was low.

Influenza background information:

Influenza is a viral infection characterized by fever, chills, malaise, headaches, body aches, and cough, sometimes complicated by pneumonia, which may be life-threatening. All age groups may be affected, but severe illness is more common in the elderly and in those with chronic illnesses such as asthma, diabetes, kidney failure, and heart disease. Influenza occurs in annual epidemics from November to March in the temperate regions of the Northern Hemisphere and from April to September in the temperate regions of the Southern Hemisphere. However, travelers in large groups, especially those on cruise ships, may be at risk year-round, due to exposure to influenza viruses carried by persons from other parts of the world. Influenza is reported sporadically throughout the year in the tropics.

Because influenza may cause significant distress during foreign travel, because medical care may be difficult to obtain while abroad, and because the symptoms of influenza, which are non-specific and may be confused with those of other illnesses, influenza vaccine is recommended for all those greater than six months of age traveling to areas where influenza may be transmitted, including the Northern Hemisphere between November and March, the Southern Hemisphere between April and September, and the tropics at any time, as well as those traveling on cruise ships. Influenza vaccine is particularly important for all those over age 65 and for those with chronic medical conditions such as diabetes, emphysema, asthma, or heart disease. The vaccine should be given at least two weeks before departure.

Twice each year, before influenza season in the Northern and Southern Hemispheres, the World Health Organization makes recommendations for vaccine composition, depending upon which strains appear most likely to cause outbreaks. If influenza vaccine for the Southern Hemisphere is not available, the vaccine for the preceding influenza season in the Northern Hemisphere, if obtainable, is the recommended alternative. For the year 2009, the vaccine recommended for the Southern Hemisphere is the same as that which had been given in the Northern Hemisphere for the winter of 2008-2009.

The most frequent side-effect of influenza vaccine is mild discomfort at the injection site. Fever, malaise, and body aches may occur, but are typically mild. Severe reactions, generally allergic, are rare. Because the viruses in the vaccine are inactivated, influenza vaccine cannot cause influenza. Influenza vaccine should not be given to anyone allergic to eggs or in the first trimester of pregnancy.

A new nasal-spray flu vaccine was licensed in the United States in 2003. Unlike the injectable flu vaccine, it contains live, weakened flu virus. It includes the same strains of influenza as the injectable vaccine and appears to have comparable efficacy. In the United States, the nasal-spray vaccine is only approved for use in healthy people between the ages of 5 and 49.

The first-line drugs to treat influenza have been oseltamivir (Tamiflu) and zanamivir (Relenza) (PDF). However, in December 2008, the Centers for Disease Control reported that almost all the strains of influenza A (H1N1) isolated to date that winter were resistant to oseltamivir (Tamiflu). In March 2009, the World Health Organization reported high prevalence of oseltamivir resistance among H1N1 strains from Canada, Hong Kong SAR, Japan, the Republic of Korea, the United States, France, Germany, Ireland, Italy, Sweden and the United Kingdom. The CDC has therefore advised that, when influenza A (H1N1) virus infection or exposure is suspected, zanamivir or a combination of oseltamivir and rimantadine (Flumadine) should be given, rather than oseltamivir alone (see the Centers for Disease Control. Oseltamivir is available as 75-mg capsules, given twice daily by mouth for five days. The most common side-effects are nausea and vomiting, which are generally mild. Zanamivir is a dry powder prepared as an oral inhaler, given two inhalations twice daily for five days. Zanamivir may cause an exacerbation of asthma or chronic obstructive lung disease; it should be given with caution to persons suffering from those diseases. The usual dosage of rimantadine is 100 mg twice daily. Rimantadine should be avoided in those with a history of seizures, though the risk appears small. If rimantadine is not available, an older drug called amantadine is an acceptable alternative. Rimantadine and amantadine are only effective against influenza A viruses, as opposed to oseltamivir and zanamivir, which have been active against both influenza A and B.

From the World Health Organization

Influenza fact sheet

Influenza vaccine

Influenza vaccines - WHO position paper (PDF)

From the Centers for Disease Control (CDC)

Influenza Home Page

Influenza

Influenza: The Disease

Influenza: Prevention and Control

Influenza: Questions and Answers

Influenza: Vaccine Information

Antiviral Drugs for Influenza

Influenza B Virus Outbreak on a Cruise Ship --- Northern Europe, 2000

Chiefly for physicians:

Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2011

Neuraminidase Inhibitors for Treatment of Influenza A and B Infections (MMWR December 17, 1999/Vol. 48/RR-14

From Health Canada

Supplementary Statement for the 2002-2003 Influenza Season:Update on Oculo-Respiratory Syndrome in Association with Influenza Vaccination

From Emerging Infectious Diseases

Special Issue: Influenza