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, although globally there are two flu seasons because winter occurs at different times of the year in the northern and southern hemispheres. A typical year's normal two flu seasons result in three to five million cases of severe illness and up to 500,000 deaths worldwide, which by some definitions is a yearly influenza epidemic. 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

Influenza in humans is typically transmitted person to person, though in the event of a mutation the virus can spread from an animal to a human host (thus “swine flu” or “bird flu”). In general, influenza is transmitted from infected hosts through the air by coughs or sneezes that create aerosols (i.e., airborne water droplets) containing the virus, and from infected birds through their droppings. Influenza can also be transmitted by saliva, nasal secretions, feces, and blood. Infections occur either through direct contact with these bodily fluids or by contact with contaminated surfaces.

Signs and symptoms

The symptoms of influenza infection include nonproductive cough, high-grade fever (102°F or more), sore throat, nasal congestion, tiredness, and headache. Nausea and vomiting sometimes occur, usually in children.

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 (under 2 years old) and the elderly (over 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®). As these drugs are somewhat limited in supply, it is recommended that their use be prioritized for medical personnel and those suffering from or at risk of serious complication from influenza. As with most diseases, prevention is still the easiest and most efficient way of mitigating the effects of an influenza outbreak.

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, an influenza vaccine is only effective for a few years at best, and it is possible to receive the vaccine and still contract influenza. The vaccine is reformulated each season for a few specific 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. If a future outbreak proves to be both highly infectious and virulent, 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.

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. The majority of 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. The spread of the disease was greatly aided by the war, which, due to the cramped living conditions of the troops and the international travel involved in their transport, gave the virus an ideal ecology in which to reproduce. Since that time, there has not been an outbreak to match it for prevalence or deadliness, but because of the mutation rate of influenza and the modern rate of global travel, it is likely only a matter of time before such a strain emerges. 

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

Strategies to improve 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 pandemic 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.

PATH is currently conducting a study to assess worldwide ability to produce vaccines in the event of a pandemic flu. One goal of the analysis is to identify strategies that will build the capacity of developing-country manufacturers to become flu vaccine suppliers, thus accelerating the availability of vaccines to the developing world.