Updated: April 21, 2010
Influenza viruses are grouped into 3 major types (A, B, and C), and strains are further divided into multiple subtypes depending on the source of the virus and the types of proteins on the outside of the virus particle.
Although influenza B causes many children to be absent from school, influenza A viruses cause much of the severe illness during epidemics. All influenza viruses quickly change as people become immune to the strains circulating in the population so that immunity acquired one year will only partially protect for subsequent outbreaks.
Influenza A viruses also have a remarkable potential for variation, mutating rapidly and also by being able to mix genetic material with influenza viruses from other species of birds and animals.
Influenza viruses can infect anyone, but rates of infection are highest among children. Serious illnesses and death also occur in all age groups but rates are greatest in persons over the age of 65 years and those who have chronic health problems. Epidemics of influenza occur during the winter months nearly every year. Influenza is spread through coughing and sneezing, and is highly contagious, especially in childcare centers, schools, and nursing homes. Annual immunization and frequent hand washing are the easiest ways to prevent infection.
Uncomplicated influenza generally comes on suddenly, and symptoms include muscle aches, fever, chills, headache, cough, and runny nose; it lasts for 3-7 days although cough can persist for about 2 weeks. The respiratory illnesses caused by influenza viruses are clinically difficult to distinguish from the illnesses caused by other respiratory infections. Young infants may have symptoms that mimic invasive bacterial infections with high fevers and fussiness, leading to hospitalization. Although most young children who are hospitalized with influenza virus infections are only in the hospital for a few days, some require treatment in an intensive care unit. The majority of children who are hospitalized for influenza infection are less than 5 years of age and a quarter of them are less than 6 months old. Influenza viruses can cause viral pneumonia, can make underlying medical conditions worse, and can lead to bacterial pneumonia, sinusitis and ear infections. Influenza virus infections have also been associated with inflammation of the heart as well as brain swelling with liver failure.
More than 90% of influenza-related deaths are in people aged 65 years or older. Between 1957 and 1986 there were 19 different annual influenza epidemics in the US; causing as many as 40,000 deaths per year, mostly among older people and those with other health problems, accounting for $1-3 billion in direct costs for medical care/year in the United States. Although influenza-related deaths are usually less common in children than the elderly, fatal cases in children have been increasingly recognized.
An influenza pandemic can occur when a new influenza A virus appears against which the human population has no immunity and when the new virus can spread from person to person. This can result in several, and simultaneous epidemics around the world. These virus strains may result from exchange of gene segments between human and avian or swine influenza viruses or from direct transmission of nonhuman viruses to humans.
The severity of pandemics has varied substantially in the past. Depending on the virulence of the new virus—the degree of disease severity it causes—the numbers of deaths, hospitalizations, work that cannot be performed, and school absenteeism—can differ a great deal from pandemic to pandemic.
During the 20th century, there were three major pandemics; the worst caused 21 million deaths worldwide and 500,000 deaths in the U.S. during 1918 and 1919, with many deaths among young adults. The other two pandemics occurred in 1957, causing 69,800 deaths, and in 1968, with 33,800 deaths, with the greatest risk of death being the traditional high risk groups: those older than 64 years and those with underlying medical conditions.
Bioterrorism, emerging infectious diseases—like SARS a few years ago—and influenza pandemics have something in common: We can predict that they will happen but we don’t know when. This is the reason that scientists have been monitoring so closely for new strains of influenza viruses that have not been seen in humans before.
Just because a new virus develops does not mean that a pandemic will occur, however. There have been many pandemic “false alarms”. But when a new strain emerges and it spreads readily from person to person, it has the potential to cause a pandemic—that is, to spread worldwide. It then becomes critical to determine how severe infection is likely to be in order to predict the likely severity of a pandemic and to guide the implementation of public health plans. A new pandemic influenza virus emerged in 2009, which contained genetic information from swine, avian and human influenza viruses. This virus has had a number of names since its recognition; its proper scientific name is A/California/07/2009 (H1N1) but it is generally called pH1N1 (2009) (the ‘p’ is for pandemic). Although this strain bears all the characteristics of a pandemic influenza virus, so far it has proven to cause less frequent severe disease than was feared might happen. While not as severe during the first season as had been feared, this strain did cause a disproportionate number of illnesses and deaths in children, pregnant women and among those with underlying health problems such as asthma and (a newly recognized risk group) those with obesity. The seasonal influenza vaccine for 2010/2011 will include this strain because it is widely believed that this strain will return as the predominant strain in the 2010/2011 influenza season.
As the pH1N1 (2009) strain circulates in the southern hemisphere this summer, scientists will watch its epidemiology and track the illnesses it causes in hopes of learning whether it will return to North Americas as a more virulent (causing more severe disease) virus than this past year and to see whether it is beginning to change—the way that influenza viruses do—as people become more immune.
Scientists continue to monitor the H5N2 avian influenza virus in other parts of the world, as well. That strain is highly virulent (the ability to cause severe illness) but has not been highly communicable among people so far.
The best protection from both seasonal and pandemic influenza is immunization. Over the past decades, influenza vaccine recommendations have stressed the importance of vaccinating those with high risk conditions that place them at increased risk from the complications of influenza infection. Historically, with influenza vaccine as well as other vaccines, that strategy hasn’t proven to be as successful as immunizing everyone. Partly this is because, with highly communicable diseases like influenza, the people who are most likely to become infected and to spread the infection are children and most or close to all of them need to be immune to get community (herd) immunity.