Overview

Influenza is a virus that causes mild to severe respiratory illness; in vulnerable populations, it can occasionally lead to death. The virus is at its peak in winter in temperate climates. Seasonality of influenza is poorly defined in tropical regions. In tropical countries where surveillance data are available, influenza viruses may be both endemic and epidemic—that is, viruses circulate year-round, with peaks in influenza during one or more time periods. Globally, influenza results in three to five million cases of severe illness and up to 500,000 deaths worldwide in a typical year. In the United States alone, the annual flu season causes illness in 5 percent to 20 percent of the population, more than 200,000 hospitalizations, and 36,000 deaths.

Transmission

In general, flu viruses are spread from person to person through coughing or sneezing of people with influenza. Sometimes people may become infected by touching something with flu viruses on it and then touching their mouth or nose. In rare cases, the virus can spread from an animal to a human host. For example, most cases of avian flu are spread directly from birds to people.

Signs and symptoms

The symptoms of influenza infection include fever, cough, sore throat, nasal congestion, tiredness, and headache. Nausea and vomiting sometimes occur.

Influenza usually resolves itself after three to seven days, though a cough may persist for two weeks or more. However, among certain persons, influenza can lead to secondary bacterial or viral pneumonia, or it can occur as part of a co-infection with other viruses or bacteria. Young children (less than two years old) and older persons (more than 65 years old) are especially susceptible to such complications, as are people with compromised immune systems or underlying medical conditions such as lung or heart disease. Respiratory illness caused by influenza can be difficult to distinguish from illness caused by other pathogens.

Treatment

For most people, it is sufficient to rest and treat the symptoms of influenza with over-the-counter medication. In addition, there are four prescription antivirals on the market licensed for treatment of the flu: amantadine (Symmetrel®), rimantadine (Flumadine®), zanamivir (Relenza®), and oseltamivir (Tamiflu®). In light of the current influenza A(H1N1) outbreak, the virus being isolated is susceptible to two of these antiviral medications—Relenza and Tamiflu. Current recommendations and information on the availability of these medications are updated frequently on the US Centers for Disease Control website and the World Health Organization website.

Prevention

Vaccination is the best means of preventing the transmission of influenza, and it is strongly recommended that vulnerable populations (such as young children and the elderly) receive the vaccine every year. Due to the rapid mutation rate of the virus, influenza vaccine must be given annually. The vaccine is reformulated each season for three flu strains, but it cannot possibly include all the strains actively infecting people in the world for that season.

In addition to the difficulties relating to strain prediction, there is currently a significant lack of manufacturing capacity for new vaccines. It will prove difficult to produce enough vaccine in time to protect significant portions of the population. International public health leaders are now working to address this imbalance.

In the case of the current outbreak of influenza A(H1N1), the best way to control the spread of the virus until an effective vaccine can be produced is through personal hygiene (such as handwashing and wearing masks), staying away from work or school when sick, and avoiding crowded gatherings, especially in confined conditions.

A look into the past and into the future

In 1918, near the end of the First World War, a highly infectious and extremely virulent strain of influenza triggered a global pandemic that is estimated to have killed between 50 and 100 million people. Many of the deaths were from bacterial pneumonia, a secondary infection that usually set in after the virus had weakened the immune system and damaged the lungs—but the virus also killed people directly, causing massive hemorrhages and edema (fluid) in the lungs. Since that time, there has fortunately not been an outbreak to match it for prevalence or deadliness.

Governments and international bodies like the World Health Organization are already implementing efforts to minimize the impact of a potential major pandemic. The work underway includes efforts to develop new vaccines, stockpile existing vaccines, increase manufacturing capacities, and reinforce health systems.

Access to influenza vaccines

The best way to control the spread of influenza is through the use of vaccines. The rapid development, production, and distribution of influenza vaccines could potentially save millions of lives during an influenza pandemic, but the production of vaccines for use in developing countries is unlikely to occur if left solely to marketplace forces. In addition, seasonal influenza remains a disease that is largely unaddressed in the developing world.

In 2007, PATH conducted a study to assess worldwide ability to produce vaccines in the event of a pandemic flu. The report analyzes the current supply of influenza vaccines, the predicted demand for influenza vaccines in case of a pandemic, and new vaccine technologies that could provide for the world’s demand in a pandemic situation. In light of the report’s findings, PATH is collaborating with public- and private-sector partners to advance the development of promising new vaccines that could more easily be used in case of an influenza pandemic.