Prevention and control of influenza: The influenza recommendations include new and updated information. Vaccination efforts should begin as soon as the seasonal influenza vaccine is available and continue through the influenza season.
These recommendations also include a summary of safety data for U. These recommendations and other information are available at CDC's influenza website http: Recommendations for influenza diagnosis and antiviral use will be published before the start of the influenza season.
Vaccination and health-care providers should be alert to announcements of recommendation updates and should check the CDC influenza website periodically for additional information.
Introduction In the United States, annual epidemics of 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 Influenza epidemics were associated with estimated annual averages of approximately 36, deaths during and approximately , hospitalizations during 6,7.
Influenza A subtypes that are generated by a major genetic reassortment i. In April , a novel influenza A H1N1 virus, influenza A H1N1 , that is similar to but genetically and antigenically distinct from influenza A H1N1 viruses previously identified in swine, was determined to be the cause of respiratory illnesses that spread across North America and were identified in many areas of the world by May 9, Influenza morbidity caused by pandemic influenza A H1N1 remained above seasonal baselines throughout spring and summer and was the cause of the first pandemic since In the United States, the pandemic was characterized by a substantial increase in influenza activity, as measured by multiple influenza surveillance systems, that was well beyond historical norms in September , peaking in late October , and returning to seasonal baseline by January Figures 1 and 2.
Data from epidemiologic studies conducted during the influenza A H1N1 pandemic indicate that the risk for influenza complications among adults aged years who had pandemic influenza A H1N1 was greater than typically occurs for seasonal influenza Influenza caused by pandemic influenza A H1N1 virus is expected to continue to occur during future winter influenza seasons in the Northern and Southern Hemispheres, but whether pandemic influenza A H1N1 viruses will replace or co-circulate with one or more of the two seasonal influenza A virus subtypes seasonal H1N1 and H3N2 that have co-circulated since is unknown.
Influenza viruses undergo frequent antigenic change as a result of point mutations and recombination events that occur during viral replication i. The extent of antigenic drift and evolution of pandemic influenza A H1N1 virus strains in the future cannot be predicted. Annual influenza vaccination is the most effective method for preventing influenza virus infection and its complications 8.
Annual vaccination with the most up-to-date strains predicted on the basis of viral surveillance data is recommended. Live, attenuated influenza vaccine LAIV may be used for healthy nonpregnant persons aged 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.
Although vaccination coverage has increased in recent years for many groups recommended for routine vaccination, considerable room for improvement remains 13 , and strategies to improve vaccination coverage in the medical home and in nonmedical settings 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 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.
CDC has revised recommendations for antiviral treatment and chemoprophylaxis of influenza periodically in response to new data on antiviral resistance patterns among circulating strains and risk factors for influenza complications With few exceptions, pandemic influenza A H1N1 virus strains that began circulating in April remained sensitive to oseltamivir As the Work Group reviews the annual recommendations for consideration by the full ACIP, its members discuss a variety of issues, including the burden of influenza illness; vaccine effectiveness, vaccine safety, and coverage in groups recommended for vaccination; feasibility; cost-effectiveness; and anticipated vaccine supply.
Work Group members also request periodic updates on vaccine and antiviral production, supply, safety, and efficacy from vaccinologists, epidemiologists, and manufacturers.
State and local vaccination program representatives are consulted. CDC's Influenza Division available at http: The Vaccines and Related Biological Products Advisory Committee provides advice on vaccine strain selection to the Food and Drug Administration FDA , which selects the viral strains to be used in the annual trivalent influenza vaccines.
Published, peer-reviewed studies are the primary source of data used by ACIP in making recommendations for the prevention and control of influenza, but unpublished data that are relevant to issues under discussion also are considered.
Among studies discussed or cited, those of greatest scientific quality and those that measure influenza-specific outcomes are the most influential. For example, population-based estimates of influenza disease burden supported by laboratory-confirmed influenza virus infection outcomes contribute the most specific data. The best evidence for vaccine or antiviral efficacy comes from randomized controlled trials that assess laboratory-confirmed influenza infections as an outcome measure and consider factors such as timing and intensity of influenza viruses' circulation and degree of match between vaccine strains and wild circulating strains 17, However, randomized controlled trials cannot be performed ethically in populations for which vaccination already is recommended, and in this context, observational studies that assess outcomes associated with laboratory-confirmed influenza infection also can provide important vaccine or antiviral safety and effectiveness data.
Evidence for vaccine or antiviral safety also is provided by randomized controlled studies; however, the number of subjects in these studies often is inadequate to detect associations between vaccine and rare adverse events.
The best way to assess the frequency of rare adverse events after vaccination is by controlled studies after vaccines are used widely in the population. These studies often use electronic medical records from large linked clinical databases and medical charts of persons who are identified as having a vaccine adverse event Finally, studies that assess vaccination program practices that improve vaccination coverage are most influential in formulating recommendations if the study design includes a nonintervention comparison group.
Data presented in this report were current as of June 29, , and represent recommendations presented to the full ACIP and approved on February 24, , and June 24, Modifications were made to the ACIP statement during the subsequent review process at CDC to update and clarify wording in the document. Further updates, if needed, will be posted at CDC's influenza website http: Primary Changes and Updates in the Recommendations The recommendations include five principal changes or updates: This represents an expansion of the previous recommendations for annual vaccination of all adults aged years and is supported by evidence that annual influenza vaccination is a safe and effective preventive health action with potential benefit in all age groups.
The only group remaining that was not recommended for routine vaccination was healthy nonpregnant adults aged years who did not have an occupational risk for infection and who were not close contacts of persons at higher risk for influenza-related complications.
However, some adults who have influenza-related complications have no previously identified risk factors for influenza complications. In addition, some adults who have medical conditions or age-related increases in their risk for influenza-related complications or another indication for vaccination are unaware that they should be vaccinated.
Further support for expansion of annual vaccination recommendations to include all adults is based on concerns that pandemic influenza A H1N1 -like viruses will continue to circulate during the influenza season and that a substantial proportion of young adults might remain susceptible to infection with this virus.
Data from epidemiologic studies conducted during the pandemic indicate that the risk for influenza complications among adults aged years is greater than is seen typically for seasonal influenza 12,23, As in previous recommendations, all children aged 6 months years who receive a seasonal influenza vaccine for the first time should receive 2 doses. Children who received only 1 dose of a seasonal influenza vaccine in the first influenza season that they received vaccine should receive 2 doses, rather than 1, in the following influenza season.
In addition, for the influenza season, children aged 6 months years who did not receive at least 1 dose of an influenza A H1N1 monovalent vaccine should receive 2 doses of a seasonal influenza vaccine, regardless of previous influenza vaccination history.
Children aged 6 months years for whom the previous seasonal or influenza A H1N1 monovalent vaccine history cannot be determined should receive 2 doses of a seasonal influenza vaccine.
Background and Epidemiology 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: Influenza A subtypes and B viruses are separated further into groups on the basis of antigenic similarities.
New influenza virus variants result from frequent antigenic change i. Recent studies have explored the complex molecular evolution and epidemiologic dynamics of influenza A viruses New or substantially different 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 when little or no previously existing immunity has been identified among humans 8.
In April , human infections with a novel influenza A H1N1 virus were identified, and this virus subsequently caused a worldwide pandemic 9. The pandemic influenza A H1N1 virus is derived from influenza A viruses that have circulated in swine during the past several decades and is antigenically distinct from human influenza A H1N1 viruses in circulation since The pandemic influenza A H1N virus contains a combination of gene segments that had not been reported previously in animals or humans.
The hemagglutination HA gene, which codes for the surface protein most important for immune response, is related most closely to the HA found in contemporary influenza viruses circulating among pigs. This HA gene apparently evolved from the avian-origin pandemic influenza H1N1 virus, which is thought to have entered human and swine populations at about the same time 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 surface antigens, particularly 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. The pandemic influenza A H1N1 virus is not a new subtype, but because most humans had no pre-existing antibody to key pandemic influenza A H1N1 virus hemagglutinin epitopes, widespread transmission was possible Health-Care Use, Hospitalizations, and Deaths Attributed to Influenza In the United States, annual epidemics of influenza typically occur during the fall or winter months, but the peak of influenza activity can occur as late as April or May.
Influenza-related complications requiring urgent medical care, including hospitalizations or deaths, can result from the direct effects of influenza virus infection, from complications associated with age or pregnancy, or from complications of underlying cardiopulmonary conditions or other chronic diseases.
Studies that have measured rates of a clinical outcome without a laboratory confirmation of influenza virus infection e. However, increases in health-care provider visits for acute febrile respiratory illness occur each year during the time when influenza viruses circulate.
Data from the U. In typical winter influenza seasons, an increase in deaths and hospitalizations is observed during periods when influenza viruses are circulating. Some persons whose hospitalization is attributed to invasive pneumococcal pneumonia are likely to have influenza as a co-pathogen, based on correlation between influenza activity and seasonal variations in pneumococcal pneumonia The number of deaths or hospitalizations attributable at least partly to influenza can be estimated by applying modeling techniques to viral surveillance and national mortality or hospitalizations data and includes deaths and hospitalizations for which influenza infection is likely a contributor to mortality but not necessarily the sole cause of death 6,7,40, Excess deaths and hospitalizations during influenza season that are likely to be caused at least partly by influenza are derived from the broad category of pulmonary and circulatory deaths or hospitalizations.
Estimates that include only outcomes attributed to pneumonia and influenza underestimate the proportion of severe illnesses that are attributable at least partly to influenza because such estimates exclude deaths caused by exacerbations of underlying cardiac and pulmonary conditions that are associated with influenza infection 6,7, During seasonal influenza epidemics from through , the estimated annual overall number of influenza-associated hospitalizations in the United States ranged from approximately 55, to , per annual epidemic mean: In the United States, the estimated number of influenza-associated deaths increased during When mortality data that included deaths attributable to both the pneumonia and influenza as well as the respiratory and circulatory categories were used as a basis for estimating the influenza burden, an average of approximately 19, influenza-associated deaths per influenza season occurred during compared with an average of approximately 36, deaths per season during 6.
On the basis of data from the pneumonia and influenza category alone, an estimated annual average of 8, influenza-related deaths occurred. From the influenza season through the season, the estimated annual number of deaths attributed to influenza ranged from 17, to 51, per epidemic 6.
Estimates of mortality using a variety of different modeling techniques generally have been similar, although estimates for more recent years, when influenza A H1N1 viruses have predominated more often, have been somewhat lower Influenza viruses cause disease among persons in all age groups Estimated rates of influenza-associated hospitalizations and deaths varied substantially by age group in studies conducted during different seasonal influenza epidemics.
During , estimated average rates of influenza-associated pulmonary and circulatory deaths per , persons were 0. In one analysis, the mean age among persons who died in the United States during May--December and who had laboratory-confirmed influenza was 37 years.
In contrast, the estimated mean age among persons who died from seasonal influenza during was 76 years This difference was attributed to a lower risk for infection 51 associated with a higher prevalence of partial or full immunity among older persons, presumably as a result of exposures to antigenically similar influenza A viruses that circulated in the early-mid 20th century.
One indication of some degree of preexisting immunity was the presence of cross-reacting antibody present among approximately one third of older adults 52 , which has been attributed to similarities in the structure of the hemagglutinin protein among the H1N1 virus and those that circulated earlier in the 20th century