Immunization Issues

Seasonal and Pandemic Influenza

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Updated: April 29, 2009

Influenza viruses

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.

Seasonal influenza

Annually, influenza is responsible for an average of 20,000 deaths, 114,000 hospitalizations, and $1 billion to $3 billion in direct costs for medical care in the United States.

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.

Although influenza-related deaths are much less common in children than the elderly, fatal cases have been increasingly recognized.

Pandemic influenza

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 influenza virus that contains genetic information from swine, avian, and human influenza viruses has emerged recently. Known as swine-origin influenza A (H1N1), this virus is unrelated to any previously recognized human virus, is clearly being spread from human-to-human, and has health authorities at heightened stages of alert because of its potential to become a pandemic strain. However, the evolving information has been conflicting as to how virulent this strain may be: Mexican authorities have been reporting increased mortality (including among young adults) but illness observed to date in the US, Canada, and New Zealand appears to be less severe. You can find up-to-date information and suggestions on what you can do to stay healthy at the CDC Swine Flu site and the WHO Web site.

If an influenza pandemic occurs, we could expect the following:

• Given the high level of global traffic, the new virus may spread rapidly and may prove difficult to contain.
• Vaccines, antiviral agents and antibiotics to treat secondary infections will be in short supply; this is why CDC has established stockpiles of antiviral drugs and is developing seed viruses for vaccine development. However, it may take several months before vaccine becomes available.
• The US, like many countries and the WHO has a pandemic influenza plan and all states have a plan for local responses because
  • Medical facilities could become overwhelmed.
  • Widespread illness may result in sudden and potentially significant shortages of personnel to provide essential community services.

The effect of influenza on individual communities may be relatively prolonged when compared to other natural disasters, as it is expected that outbreaks will recur.

US Preparedness Plan

The US Department of Health and Human Services has developed a Pandemic Influenza Response and Preparedness Plan in case a pandemic influenza strain emerges. This plan includes the following key activities:

• Detect novel influenza strains through clinical and virologic surveillance of human and animal influenza disease.
• Rapidly develop, evaluate, and license vaccines against the pandemic strain and produce them in sufficient quantity to protect the population.
• Implement a vaccination program that rapidly administers vaccine to priority groups and monitors vaccine effectiveness and safety.
• Determine the susceptibility of the pandemic strain to existing influenza antiviral drugs and target use of available supplies; avoid inappropriate use to limit the development of antiviral resistance and ensure that this limited resource is used effectively.
• Implement measures to decrease the spread of disease internationally and within the U.S. guided by the epidemiology of the pandemic.
• Assist state and local governments and the health care system with preparedness planning in order to provide optimal medical care and maintain essential community services.
• Communicate effectively with the public, health care providers, community leaders, and the media.

Selected Resources (accessed 27 April, 2009)

Centers for Disease Control and Prevention

WHO, Communicable Disease Surveillance & Response (CSR). Influenza Pandemic Preparedness

Department of Health and Human Services