Influenza Season: 2010 Vaccination Guidelines

This year’s influenza season is approaching fast. Although the World Health Organization officially declared an end to the 2009 H1N1 influenza pandemic in August,¹ the H1N1 virus is still circulating and is likely to continue to cause serious disease in infants, young children, pregnant women, and other high-risk groups.

The H1N1 virus is one of 3 viruses covered by this year’s vaccine. The others are A/Perth/16/2009 (H3N2)-like and B/Brisbane/60/2008-like strains.

WHAT’S NEW IN THE 2010 VACCINATION GUIDELINES
The 2010 guidelines on influenza vaccination from the CDC’s Advisory Committee on Immunization Practices contain several notable changes from the 2009 recommendations²:
•Routine vaccination is now advised for all persons aged 6 months and older.
•Children aged 6 months to 8 years who did not receive at least 1 dose of 2009 H1N1 vaccine should receive 2 doses of 2010 seasonal influenza vaccine.
•The following inactivated influenza vaccines have been approved for use in expanded age groups: Fluarix, ages 3 years and up, and Agriflu, ages 18 years and up. Afluria should not be used in children aged 6 months through 8 years because an increased frequency of fever and febrile seizures was observed in children aged 6 months through 4 years who received an antigenically equivalent vaccine in Australia and New Zealand during the southern hemisphere influenza season.³
•Fluzone High-Dose has been approved as an alternative inactivated vaccine for persons aged 65 years and older.

A LOOK BACK AT THE H1N1 INFLUENZA PANDEMIC
In April 2009, respiratory disease caused by a novel influenza A (H1N1) virus first appeared in North America and then rapidly spread to other areas of the world. Morbidity associated with pandemic H1N1 influenza remained above seasonal baselines throughout the spring and summer of 2009. During the peak of influenza activity in the United States, more than 99% of viruses identified were the 2009 pandemic influenza A (H1N1) virus.²

H1N1 influenza is expected to continue to occur during future winter influenza seasons in the Northern and Southern Hemispheres, but whether 2009 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 1977 remains unknown.²

PERSONS AT GREATEST RISK FOR INFLUENZA COMPLICATIONS
The risks of influenza-related complications, hospitalizations, and death are increased in persons aged 65 years and older, infants and young children, and persons with underlying medical conditions. Although complications of influenza are generally rare among healthy young adults, pregnant women are also at increased risk.

The incidence of influenza complications among adults aged 19 to 64 years who had 2009 pandemic influenza A (H1N1) was greater than is typically seen with seasonal influenza. In a study that focused on the outbreak in Mexico, almost 87% of deaths and 71% of cases of severe pneumonia occurred in patients between the ages of 5 and 59 years.⁴

Among the significant complications that can occur after influenza are⁵:
•Primary viral pneumonia or mixed viral/bacterial pneumonia.
•Other secondary respiratory tract complications, including head and neck syndromes (sinusitis, otitis media, croup); bronchiolitis; asthma exacerbations; bacterial pneumonia (with secondary causative organisms, such as Streptococcus pneumoniae or group A -hemolytic streptococci); and infection with Staphylococcus aureus or Haemophilus influenzae.
•Sepsis and toxic shock syndromes.
•Carditis (either myocarditis or pericarditis).
•Neurological conditions (Guillain-Barr syndrome [GBS], aseptic meningitis, postinfectious encephalopathy, seizures, status epilepticus, myelitis).
•Myositis and rhabdomyolysis.
•Reye syndrome (especially in children who are given aspirin).