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 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 the coughs or sneezes of an infected person. 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 host to a human. 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. However, for certain cases of flu or in certain populations, antiviral medications may be appropriate. There are four prescription antivirals on the market licensed for treatment of the flu: amantadine (Symmetrel®), rimantadine (Flumadine®), zanamivir (Relenza®), and oseltamivir (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 (WHO) website.

Prevention

Vaccination is the best means of preventing influenza illness, and in the United States, everyone is recommended to receive the influenza vaccine every year. In other countries, vaccine is recommended for high risk populations such as pregnant women, young children, the elderly, and persons of all ages with certain medical conditions. Due to the rapid mutation rate of the virus, influenza vaccine must be given annually. The vaccine is reformulated twice annually—for the Northern and Southern hemisphere flu seasons—in an attempt to include the most likely circulating strains. But the vaccine strains do not always match the circulating strains.

In addition to the difficulties related to strain prediction, there is currently a significant lack of manufacturing capacity for new vaccines in many places in the world. As the recent influenza A (H1N1) pandemic illustrated, producing enough vaccine in time to protect significant portions of the population is difficult. International public health leaders are now working to address this imbalance.

A way to control the spread of the virus, in addition to an effective vaccine, is through personal hygiene (such as washing hands 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.

The influenza A (H1N1) pandemic, the first of the 21st century, was not as virulent as the 1918 pandemic. However, the virus spread quickly throughout the world and caused severe disease, particularly in certain high risk populations such as pregnant women and young children. Governments and international bodies like WHO worked together to distribute vaccine to countries that were not able to manufacture it on their own. Continued efforts are underway to develop new vaccines, stockpile existing vaccines, increase manufacturing capacities, and reinforce health systems so that the world will be better prepared for the next pandemic.

Access to influenza vaccines

The best way to control the spread and complications 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 analyzed the supply of influenza vaccines at the time, 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. While recent survey results from WHO have shown a substantial increase in flu vaccine production capacity more than three years later, in 2010, some regions of the world still do not have adequate capacity to manufacture influenza vaccines—leaving global production capacity short in a pandemic situation. PATH is collaborating with public- and private-sector partners to advance the development of promising new vaccines that could more easily be used in a future influenza pandemic.