Influenza Update - October 2005

Influenza epidemics typically occur during the winter months in temperate regions and cause disease among all age groups, but rates of infection are highest among children. Rates of serious illness and death are highest among persons aged > 65 years, children aged < 2 years, and persons of any age who have medical conditions that place them at increased risk.

The 1918 Influenza is estimated to have killed at least 20 million people worldwide(1). With ever-increasing poultry infections, intercontinental travel and the 2004 vaccine shortage, the stage may be set for the widespread proliferation of influenza. The world differences between 1918 and 2005 are dramatic, but since then, the only real tool against the flu that we have is the influenza vaccine. Antiviral drugs used for chemoprophylaxis or treatment of influenza are a key adjunct to vaccine, but not a substitute.


Clinical Signs and Symptoms of Influenza
Influenza viruses are spread from person to person primarily through the coughing and sneezing of infected persons. The typical incubation period for influenza is 1-4 days. Children can be infectious for
> 10 days.

Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms (e.g., fever, myalgia, headache, malaise, nonproductive cough, sore throat and rhinitis). Among children, otitis media, nausea and vomiting are also commonly reported with influenza illness(2). Influenza illness typically resolves after 3-7 days, although cough and malaise can persist for > 2 weeks(3). Influenza infection has also been associated with encephalopathy, transverse myelitis, Reye syndrome, myositis, myocarditis and pericarditis.

Hospitalizations and Deaths from Influenza
Among children 0-4 years, hospitalization rates range from approximately 500/100,000 children for those with high-risk medical conditions to 100/100,000 in otherwise healthy children. Hospitalization rates are highest among children 0-1 years and are comparable to rates reported among persons aged > 65 years. Deaths from influenza are uncommon among both children with and without high-risk conditions, but do occur.

Options for Controlling Influenza
The primary option for reducing the effect of influenza is immunoprophylaxis with vaccine. Inactivated (i.e., killed virus) influenza vaccine and live, attenuated influenza vaccine (LAIV) are available for use in the U.S. Vaccinating persons at high risk for complications and their contacts each year before the flu season is the most effective means of reducing the effect of influenza.

Vaccination Recommendations
Both the inactivated influenza vaccine and the LAIV can be used to reduce influenza risk. LAIV is administered intra-nasally and approved for use among healthy persons aged 5-49 years. It is not approved for those with underlying chronic diseases. Inactivated influenza vaccine is approved for persons aged > 6 months. Vaccination is recommended for children(5):

aged 6 – 23 months, on long-term aspirin therapy, with compromised respiratory function; and chronic metabolic, pulmonary or cardiac diseases

Because children aged 0-23 months are at increased risk for influenza-related hospitalization(5), vaccination is recommended for their household contacts and out-of-home caregivers. This is particularly true for contacts of children aged 0-5 months as the influenza vaccines are not FDA approved for use in children aged < 6 months. Women who will be pregnant during the influenza season should be vaccinated during any trimester. Influenza vaccine is safe for mothers who are breastfeeding and their infants.

In addition to the target groups, physicians should administer influenza vaccine to those who wish to reduce their likelihood of becoming ill with influenza or transmitting influenza to others should they become infected, depending on vaccine availability. Students or other persons in institutional settings (e.g., those who reside in dormitories) should be encouraged to receive vaccine(6).

Individuals with the following conditions should not receive a flu shot:
1) A past severe reaction to a flu shot;
2) Severe allergy to hens’ eggs;
3) Had Guillain-Barre syndrome < 6 weeks after a flu shot.

Inactivated Influenza Dosing Recommendations
For adults and older children, the recommended vaccination site is the deltoid muscle. For infants and young children, the preferred site is the anterolateral aspect of the thigh. Two doses administered at least 1 month apart are recommended for children aged < 9 years who are receiving their first influenza vaccine (See Table)(9).


References

1) Centers for Disease Control and Prevention. Information about influenza pandemics. March 2005. Available at: http://www.cdc.gov/flu/avian/gen-info/pandemics.htm. Accessed August 4, 2005.

2) Nicholson KG. Clinical features of influenza. Semin Respir Infect 1992;7:26-37.

3) Chiu SS, Tse CY, Lau YL, et al. Influenza A infection is an important cause of febrile seizures. Pediatrics 2001:108:E63.

4) Centers for Disease Control and Prevention. Prevention and control of influenza. July 2005. Available at http://www.cdc.gov/mmwr/preview/mmwr.html. Accessed August 4, 2005.

5) Izurieta HS, Thompson WW, Kramarz P, et al. Influenza and the rates of hospitalization for respiratory disease among infants and young children. N Engl J Med 2000;342:232-239.

6) Centers for Disease Control and Prevention. Prevention and control of influenza. July 2005. Available at http://www.cdc.gov/mmwr/preview/mmwr.html Accessed August 4, 2005.

This information provided by Colleen Panina, M.D. and the Pediatric Hospitalist Team
Tel (415) 809-9159