ATLANTA-- A substantial delay in the distribution of influenza vaccine and a possible shortage in its supply for the 2000-2001 influenza season are expected, according to a recent update from the Centers for Disease Control and Prevention (CDC).[1] In response to the delay, which is due to decreased production resulting from manufacturing problems, the CDC has issued adjunct recommendations for the upcoming influenza season. The update follows on the heels of the CDC's introduction of major changes to its recommendations for influenza prevention and control, which were published in an earlier report.[2]

A key change presented in that report is the lowering of the minimum adult age for universal influenza vaccination from 65 to 50 years of age. The change reflects the fact that a quarter to a third of those between the ages of 50 and 64 years have diabetes, hypertension, or other chronic conditions that put them at high risk for hospitalization and death due to influenza.

Despite the potential vaccine shortage, the CDC has not backed away from this recommendation, but it has suggested postponing the start of this year's organized vaccination campaigns. "The first line of defense against influenza is still vaccination," Kristin L. Nichol, MD, told RESPIRATORY REVIEWS. Dr. Nichol was a member of the Advisory Committee on Immunization Practices (ACIP) Influenza Working Group, the CDC committee that produced the influenza update.

Vaccination rates have largely been inadequate, according to the CDC. In 1997, for example, less than 30% of high-risk persons under age 65 years were vaccinated--a rate far below the Healthy People 2000 goal of 60%.3 Rates for blacks and Hispanics continue to trail behind those of whites.[3,4]

WHO SHOULD BE VACCINATED?

In the absence of febrile illness and allergy to influenza vaccine components, influenza vaccination is acceptable for virtually any healthy person age 6 months or older. However, the vaccine is strongly recommended for anyone age 6 months or older who is at increased risk of influenza-related complications because of age or underlying medical conditions (Table 1).

Approximately 10 to 13 million Americans between the ages of 50 and 64 years have chronic medical conditions that leave them at increased risk of serious complications from influenza. Yet, only an estimated 40% of them received flu vaccines in 1997.[4] The new recommendation for universal vaccination of persons age 50 to 64 years is aimed at protecting those with chronic conditions.

"The flu vaccine also provides a significant benefit in healthy populations, particularly for preventing the spread of the disease to those at high risk," ACIP Influenza Working Group Chairman Fernando A. Guerra, MD, MPH, told RESPIRATORY REVIEWS.

Despite the delay in the distribution of the vaccine, the CDC recommends that routine vaccination of persons at high risk for complications from influenza and their close contacts should proceed as normal with available vaccines. In other words, these persons should be vaccinated during regular health-care visits.

TIMING OF VACCINATION

Early October through mid November is usually the optimal time for influenza vaccination because influenza activity typically peaks between late December and early March. However, for the 2000-2001 season, the CDC recommends that organized influenza vaccination campaigns should be delayed until early to mid November. "Large, organized programs are very important in vaccine delivery," emphasized Dr. Nichol, who is also Chief of Medicine at the Minneapolis VA Medical Center. To minimize the number of high-risk patients who are missed, health care providers should offer these patients vaccination during hospitalization or routine care as soon as the vaccine is available.

The trivalent vaccine for the 2000-2001 influenza season will include A/Moscow/10/99(H3N2)-like, A/New Caledonia/20/99(H1N1)-like, and B/Beijing/184/93-like antigens. "The vaccine has considerable efficacy in preventing the flu or limiting disease severity in those who are exposed and do get sick," said Dr. Guerra, Director of Health of the San Antonio Metropolitan Health District in Texas. It may offer protection against mild viral respiratory tract infections.

"The vaccine is given to tens of millions of people every year and has an excellent safety profile," added Dr. Nichol. Occasionally, adverse reactions occur. Among the most common are mild injection-site soreness lasting up to two days and systemic symptoms such as fever, malaise, and myalgia starting six to 12 hours after vaccination and lasting one to two days. However, recent controlled trials in elderly persons and healthy young adults suggest that the rate of systemic symptoms is no higher following receipt of the vaccine than after placebo administration, according to Dr. Nichol.

PROPER USE OF ANTIVIRALS

Antiviral therapy can be a helpful adjunct in influenza prevention and control, but it is not a substitute for influenza vaccination. In otherwise healthy adults, antiviral therapy can reduce the duration of uncomplicated influenza by about a day when administered within two days of illness onset.

Zanamivir and oseltamivir are effective against influenza A and B, whereas the older agents (amantadine and rimantadine) are effective only against influenza A. The two older antivirals are FDA-approved for influenza prophylaxis, however. The two new ones are not.

None of these agents has been shown to prevent serious influenza-related complications, such as bacterial or viral pneumonia or exacerbation of chronic diseases, and their efficacy in high-risk groups and children is unclear. Zanamivir has been approved for use in children age 12 years and older. Oseltamivir has not been approved for pediatric use.

Furthermore, zanamivir should be used with caution in patients with chronic obstructive pulmonary disease or asthma, warned Dr. Nichol. There have been reports of a decline in respiratory function and/or bronchospasm in some of these patients after zanamivir administration.

To minimize the risk of influenza resistance to antivirals, therapy should typically be discontinued after three to five days or within 24 to 48 hours of symptom resolution in patients receiving amantadine or rimantadine. The recommended treatment duration for zanamivir and oseltamivir is five days.

--Timothy Begany

References
1. Centers for Disease Control and Prevention. Delayed supply of influenza vaccine and adjunct ACIP influenza vaccine recommendations for the 2000-2001 influenza season. MMWR Morb Mortal Wkly Rep. 2000;49:619-622.
2. Centers for Disease Control and Prevention. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Morb Mortal Wkly Rep. 2000;49:1-38.
3. Singleton JA, Lu PJ, Strikas RA. Progress toward the Healthy People 2000 influenza objective, United States, 1997. In: Abstracts of the 34th National Immunization Conference. Atlanta, Ga: CDC, 2000 (in press).
4. Centers for Disease Control and Prevention. Influenza and pneumococcal vaccination levels among adults aged 65 years or older. MMWR Morb Mortal Wkly Rep. 1998;47:797-802.