Seasonal Influenza (Flu)

ACIP Recommendations: Introduction and Biology of Influenza

In the United States, annual epidemics of seasonal influenza occur typically during the late fall through early spring. Influenza viruses can cause disease among persons in any age group, but rates of infection are highest among children. Rates of serious illness and death are highest among persons aged 65 years and older, children aged <2 years, and persons of any age who have medical conditions that place them at increased risk for complications from influenza. An annual average of approximately 36,000 deaths during 1990—1999 and 226,000 hospitalizations during 1979--2001 have been associated with influenza epidemics.

Annual influenza vaccination is the most effective method for preventing influenza virus infection and its complications. Influenza vaccine can be administered to any person aged >6 months who does not have contraindications to vaccination to reduce the likelihood of becoming ill with influenza or of transmitting influenza to others. Trivalent inactivated influenza vaccine (TIV) can be used for any person aged 6 months and older, including those with high-risk conditions (Boxes 1 and 2). Live, attenuated influenza vaccine (LAIV) may be used for healthy, nonpregnant persons aged 2—49 years. No preference is indicated for LAIV or TIV when considering vaccination of healthy, nonpregnant persons aged 2—49 years. Because the safety or effectiveness of LAIV has not been established in persons with underlying medical conditions that confer a higher risk for influenza complications, these persons should be vaccinated only with TIV. Influenza viruses undergo frequent antigenic change (i.e., antigenic drift); to gain immunity against viruses in circulation, patients must receive an annual vaccination against the influenza viruses that are predicted on the basis of viral surveillance data. Although vaccination coverage has increased in recent years for many groups targeted for routine vaccination, coverage remains low among most of these groups, and strategies to improve vaccination coverage, including use of reminder/recall systems and standing orders programs, should be implemented or expanded.

Antiviral medications are an adjunct to vaccination and are effective when administered as treatment and when used for chemoprophylaxis after an exposure to influenza virus. However, the emergence since 2005 of resistance to one or more of the four licensed antiviral agents (oseltamivir, zanamivir, amantadine and rimantadine) among circulating strains has complicated antiviral treatment and chemoprophylaxis recommendations. Updated antiviral treatment and chemoprophylaxis recommendations will be provided in a separate set of guidelines later in 2009. CDC has issued interim recommendations for antiviral treatment and chemoprophylaxis of influenza, and these guidelines should be consulted pending issuance of new recommendations.

In April 2009, a novel influenza A (H1N1) virus that is similar to influenza viruses previously identified in swine was determined to be the cause of an influenza respiratory illness that spread across North America and was identified in many areas of the world by May 2009. The symptoms of novel influenza A (H1N1) virus infection are similar to those of seasonal influenza, and specific diagnostic testing is required to distinguish novel influenza A (H1N1) virus infection from seasonal influenza. The epidemiology of this illness is still being studied and prevention issues related to this newly emerging influenza virus will be published separately.

Biology of Influenza

Influenza A and B are the two types of influenza viruses that cause epidemic human disease. Influenza A viruses are categorized into subtypes on the basis of two surface antigens: hemagglutinin and neuraminidase. Since 1977, influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses have circulated globally. Influenza A (H1N2) viruses that probably emerged after genetic reassortment between human A (H3N2) and A (H1N1) viruses also have been identified in some influenza seasons. In April 2009, human infections with a novel influenza A (H1N1) virus were identified; as of June 2009, infections with the novel influenza A (H1N1) virus have been reported worldwide. This novel virus is derived partly from influenza A viruses that circulate in swine and is antigenically distinct from human influenza A (H1N1) viruses in circulation since 1977. Influenza A subtypes and B viruses are further separated into groups on the basis of antigenic similarities. New influenza virus variants result from frequent antigenic change (i.e., antigenic drift) resulting from point mutations and recombination events that occur during viral replication. Recent studies have begun to shed some light on the complex molecular evolution and epidemiologic dynamics of influenza A viruses.

Currently circulating influenza B viruses are separated into two distinct genetic lineages (Yamagata and Victoria) but are not categorized into subtypes. Influenza B viruses undergo antigenic drift less rapidly than influenza A viruses. Influenza B viruses from both lineages have circulated in most recent influenza seasons.

Immunity to the surface antigens, particularly the hemagglutinin, reduces the likelihood of infection. Antibody against one influenza virus type or subtype confers limited or no protection against another type or subtype of influenza virus. Furthermore, antibody to one antigenic type or subtype of influenza virus might not protect against infection with a new antigenic variant of the same type or subtype. Frequent emergence of antigenic variants through antigenic drift is the virologic basis for seasonal epidemics and is the reason for annually reassessing the need to change one or more of the recommended strains for influenza vaccines.

More dramatic changes, or antigenic shifts, occur less frequently. Antigenic shift occurs when a new subtype of influenza A virus appears and can result in the emergence of a novel influenza A virus with the potential to cause a pandemic. New influenza A subtypes have the potential to cause a pandemic when they are able to cause human illness and demonstrate efficient human-to-human transmission and little or no previously existing immunity has been identified among humans. Novel influenza A (H1N1) virus is not a new subtype, but because the large majority of humans appear to have no pre-existing antibody to key novel influenza A (H1N1) virus hemagglutinin epitopes, substantial potential exists for widespread infection

BOX 1. Summary of seasonal influenza vaccination recommendations, 2009: children and adolescents aged 6 months--18 years
All children aged 6 months--18 years should be vaccinated annually. Children and adolescents at higher risk for influenza complications should continue to be a focus of vaccination efforts as providers and programs transition to routinely vaccinating all children and adolescents, including those who: are aged 6 months--4 years (59 months); have chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, cognitive, neurologic/neuromuscular, hematological or metabolic disorders (including diabetes mellitus); are immunosuppressed (including immunosuppression caused by medications or by human immunodeficiency virus); are receiving long-term aspirin therapy and therefore might be at risk for experiencing Reye syndrome after influenza virus infection; are residents of long-term care facilities; and will be pregnant during the influenza season.
Note: Children aged < 6 months cannot receive influenza vaccination. Household and other close contacts (e.g., daycare providers) of children aged < 6 months, including older children and adolescents, should be vaccinated.

BOX 1. Summary of seasonal influenza vaccination recommendations, 2009: children and adolescents aged 6 months--18 years

All children aged 6 months--18 years should be vaccinated annually.

Children and adolescents at higher risk for influenza complications should continue to be a focus of vaccination efforts as providers and programs transition to routinely vaccinating all children and adolescents, including those who:

are aged 6 months--4 years (59 months);
have chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, cognitive, neurologic/neuromuscular, hematological or metabolic disorders (including diabetes mellitus);
are immunosuppressed (including immunosuppression caused by medications or by human immunodeficiency virus);
are receiving long-term aspirin therapy and therefore might be at risk for experiencing Reye syndrome after influenza virus infection;
are residents of long-term care facilities; and
will be pregnant during the influenza season.

Note: Children aged < 6 months cannot receive influenza vaccination. Household and other close contacts (e.g., daycare providers) of children aged < 6 months, including older children and adolescents, should be vaccinated.

BOX 2. Summary of influenza vaccination recommendations, 2009: adults
Annual vaccination against influenza is recommended for any adult who wants to reduce the risk of becoming ill with influenza or of transmitting it to others. Vaccination is recommended for all adults without contraindications in the following groups, because these persons either are at higher risk for influenza complications, or are close contacts of persons at higher risk: persons aged 50 years and older; women who will be pregnant during the influenza season; persons who have chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, cognitive, neurologic/neuromuscular, hematological or metabolic disorders (including diabetes mellitus); persons who have immunosuppression (including immunosuppression caused by medications or by human immunodeficiency virus; residents of nursing homes and other long-term care facilities; health-care personnel; household contacts and caregivers of children aged <5 years and adults aged 50 years and older, with particular emphasis on vaccinating contacts of children aged <6 months; and household contacts and caregivers of persons with medical conditions that put them at higher risk for severe complications from influenza.

BOX 2. Summary of influenza vaccination recommendations, 2009: adults

Annual vaccination against influenza is recommended for any adult who wants to reduce the risk of becoming ill with influenza or of transmitting it to others. Vaccination is recommended for all adults without contraindications in the following groups, because these persons either are at higher risk for influenza complications, or are close contacts of persons at higher risk:

persons aged 50 years and older;
women who will be pregnant during the influenza season;
persons who have chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, cognitive, neurologic/neuromuscular, hematological or metabolic disorders (including diabetes mellitus);
persons who have immunosuppression (including immunosuppression caused by medications or by human immunodeficiency virus;
residents of nursing homes and other long-term care facilities;
health-care personnel;
household contacts and caregivers of children aged <5 years and adults aged 50 years and older, with particular emphasis on vaccinating contacts of children aged <6 months; and
household contacts and caregivers of persons with medical conditions that put them at higher risk for severe complications from influenza.

NOTE: The text above is taken from Prevention & Control of Seasonal Influenza with Vaccines – Recommendations of the Advisory Committee on Immunization Practices (ACIP) 2009. MMWR 2009 Jul 24; Early Release:1–52.