Key Point:

Influenza-related hospitalization rates are high for both the very young and the very old, highlighting the need for improved vaccination in these two groups.

ATLANTA—Keeping track of annual flu-related mortality rates can help the health care community prepare for an epidemic. However, hospitalization rates are a better indicator of the total health burden caused by the flu.

Researchers from the CDC’s National Immunization Program and National Center for Infectious Diseases estimated the annual number of influenza-related hospitalizations from the 1979–1980 flu season through the 2000–2001 season, a period for which national influenza laboratory surveillance data were available. They found that influenza-related hospitalizations have increased over the study period, mostly because of the aging of the US population.¹

Influenza virus activity was reported weekly to the CDC by 50 to 75 US-based World Health Organization collaborating virology laboratories. The laboratories provided numbers of total respiratory specimens that tested positive for influenza according to virus and subtype. Monthly hospital discharge data were recorded by the National Hospital Discharge Survey for about 270,000 inpatient records from 500 hospitals. ICD-9 codes were used to categorize hospitalizations. The two diagnostic categories considered were respiratory and circulatory hospitalizations, and pneumonia and influenza (P&I) hospitalizations. P&I hospitalizations were a subset of the respiratory and circulatory diagnostic category.

RESULTS FROM THE OBSERVATION PERIOD

On average, 13.3% of specimens tested positive for influenza virus: Influenza A (H1N1) was detected in 2.1%, type A (H3N2) in 7.9%, and type B viruses in 3.3%. Of 22 influenza seasons, influenza A (H3N2) predominated 15 times, followed by type B viruses (11 times), and type A (H1N1), which predominated seven times. The total number of primary respiratory and circulatory hospitalizations decreased between the 1982 and 1991 flu seasons but increased thereafter.

During the period studied, there was an annual average of 94,735 primary hospitalizations and 133,900 any listed P&I hospitalizations that were associated with influenza viruses. These hospitalizations represented 8.6% of all primary and 8.0% of any listed P&I hospitalizations.

There was a substantial increase in hospitalizations from the 1996–1997 flu season through 1999–2000. This was a period when the type A (H3N2) virus predominated. From 2000–2001 onward, however, hospitalizations were the lowest since the 1995–1996 season—corresponding with the predominance of the type A (H1N1) virus.

People between the ages of 5 and 49 had the lowest rates of influenza-related hospitalizations. Hospitalizations increased dramatically in adults older than 49 and in children younger than 5.

TWO GROUPS AT RISK

“Even studies from the 1970s showed high rates of influenza in young children,” said Carolyn B. Bridges, MD, a Medical Epidemiologist with the CDC’s National Immunization Program. “They are very susceptible, because they don’t have preexisting antibodies.”

Children younger than 5 months will be one population who will benefit most from the vaccination of household contacts, Dr. Bridges explained. “This group is at high risk, but they cannot be vaccinated because the vaccine is not effective in children younger than 6 months. So the best way to protect them is to vaccinate the people around them.”

Another group for whom vaccination is recommended is health care workers. Even though this recommendation has been in effect for decades, “as of 2002, only 38% of health care workers had been vaccinated,” Dr. Bridge pointed out. “This certainly is a group that we need to encourage to get vaccinated; we need to provide convenient, affordable vaccination so they don’t get sick and pass on influenza to their patients.”

Influenza type A (H3N2) predominance was associated with the highest hospitalization rates (43.5/100,000 person-years for primary P&I hospitalizations and 99/100,000 person-years for primary respiratory and circulatory hospitalizations). Hospitalization rates were highest and length of hospital stay was longest for those 85 and older.

Dr. Bridges concurred, saying that overall, “the increase in the number of hospitalizations appears to be primarily due to the aging of the population.”

In general, the authors found that influenza-related hospitalization rates are increasing. This observation may be due to several factors: the aging of the population, the predominance of type A (H3N2) viruses in many recent flu seasons, and the overall trend for flu viruses to circulate for longer periods—at least during the 1990s.

A VACCINATION PROGRAM FOR CHILDREN

In October 2003, the Advisory Committee on Immunization Practices published a new recommendation for routine influenza immunization of healthy children ages 6 to 23 months. However, vaccinating this age-group requires annual vaccine administration and a follow-up dose given at least four weeks after the first dose, often during a time when parents would not normally take their children for a well-child visit. In addition, the cost of another vaccine could keep parents from having their children immunized.

Researchers at the University of Rochester, New York, and the CDC undertook a survey of physicians’ opinions regarding the feasibility of universal influenza vaccination for infants and toddlers. They found several obstacles to realizing the goal of universal immunization and outlined three strategies—minimizing costs, improving education, and streamlining the primary care system—to increase immunization rates in young children.²

A DOCTOR'S EYE VIEW

The survey was completed by 458 physicians (306 pediatricians and 152 family practitioners). Overall, an expanded recommendation for vaccinating 12- to 35-month-old children using either the injectable or the intranasal vaccine was favored by 58% of respondents, neither opposed nor favored by 23%, and opposed by 19%. The respondents expressed concern that parents would object to additional immunizations, that it could deter parents from following existing vaccination schedules, and that the live influenza vaccine might not be safe in very young children.

When asked how much more difficult it would be to implement the new vaccination scenario if only the injectable vaccine were used, 8% noted that it would be “nearly impossible,” 44% responded that it would be “much more difficult,” 37% said “slightly more difficult,” and 10% thought that it would present no additional difficulty. The perceived level of difficulty was lower if the intranasal vaccine could be used.

Common practice-based barriers to immunization included cost of vaccines, inability to identify eligible patients, the need to discuss safety concerns with parents, and the logistical difficulty of handling extra immunization visits. Nearly 75% of physicians believed that parents would see the influenza vaccine as “one more vaccine in a crowded vaccine schedule.” Another concern was the need for additional office personnel to administer the vaccine.

A bivariate analysis showed that pediatricians were more likely than family practitioners to favor an expanded recommendation, as were physicians with 100 or more patients younger than 1 year. After multivariate analysis, the variables that remained significant for favoring the expanded recommendation included the influenza disease burden for a particular season, the perceptions that immunization would decrease the overall number of illness visits, that implementing the expanded recommendation would be fairly easy, and that many parents would be interested in having their children vaccinated.

The authors concluded that to make widespread immunization of very young children feasible, three changes would be needed²:

• Minimize family out-of-pocket expense and provide reimbursement.
• Educate providers on the evidence of reduced disease burden and vaccine effectiveness and safety.
• In primary care offices, change tracking systems, institute reminder call systems, and establish immunization clinics during the vaccination season.

—Gale Jurasek

References
1. Thompson WW, Shay DK, Weintraub E, et al. Influenza-associated hospitalizations in the United States. JAMA. 2004;292:1333-1340.
2. Humiston SG, Szilagyi PG, Iwane MK, et al. The feasibility of universal influenza vaccination for infants and toddlers. Arch Pediatr Adolesc Med. 2004;158:867-874.

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