Confounding by season in ecologic studies of seasonal exposures and outcomes: examples from estimates of mortality due to influenza

PURPOSE

Many health outcomes exhibit seasonal variation in incidence, including accidents, suicides, and infections. For seasonal outcomes it can be difficult to distinguish the causal roles played by factors that also vary seasonally, such as weather, air pollution, and pathogen circulation.

METHODS

Various approaches to estimating the association between a seasonal exposure and a seasonal outcome in ecologic studies are reviewed, using studies of influenza-related mortality as an example. Because mortality rates vary seasonally and circulation of other respiratory viruses peaks during influenza season, it is a challenge to estimate which winter deaths were caused by influenza. Results of studies that estimated the contribution of influenza to all-cause mortality using different methods on the same data are compared.

RESULTS

Methods for estimating associations between season exposures and outcomes vary greatly in their advantages, disadvantages, and assumptions. Even when applied to identical data, different methods can give greatly different results for the expected contribution of influenza to all-cause mortality.

CONCLUSIONS

When the association between exposures and outcomes that vary seasonally is estimated, models must be selected carefully, keeping in mind the assumptions inherent in each model.

[Influenza vaccination and hospitalization in high risk patients]

BACKGROUND AND OBJECTIVE

We aimed to analyze the degree of utilization of influenza vaccines in patients who are 65 years and older with decompensated chronic diseases during the 2006-07 influenza epidemic and to compare it with the 2000-01 epidemic. We also investigated the effectiveness of the influenza vaccination in the prevention of incomes for cardiopulmonary illness in this group.

PATIENTS AND METHOD

Six hundred and forty two patients were studied; average (standard deviation) age was 74 (10) years, and 404 (63%) were males. Patients considered as cases had been admitted to hospital due to cardiopulmonary illness during the influenza season. Controls were patients attended at hospital but who had not been hospitalized during that period.

RESULTS

The population vaccination rate was 445 (68%). In the vaccination group, 275 (57%) people were 75 year old or more vs 77 (37%) patients in the non-vaccination group (p<0.001); 418 (94%) of the vaccination group had been vaccinated in previous years, vs 96 (46%) in the non-vaccination group (p<0.001). Cases were 337 (68%) vs 305 (31%) controls. Cases had major morbidity, previous hospitalization or pneumonia, as well as 2 or more risk factors -292 (85%) cases vs 225 (73%) controls (p<0.001)-. Factors mostly influencing the possibility of hospitalization were: influenza vaccination (odds ratio [OR]=0.59; 95% confidence interval [CI]: 0.41-0.86; p=0.006), chronic ischemic heart disease (OR=0.59; 95% CI, 0.40-0.88; p=0.009), high blood pressure (OR=1.60; 95% CI, 1.03-2.47), and previous hospitalization (OR=2.40; 95% CI, 1.64-3.49; p<0.001).

CONCLUSIONS

In high risk patients, the influenza vaccination rate was improved slightly in the last 6 years. Vaccination was associated with less hospitalization for heart and pulmonary diseases. These benefits of vaccination support future efforts to increase the rates of vaccination among elder patients with chronic diseases, especially if they have not been vaccinated in previous campaigns.

Influenza vaccination of recommended adult populations, U.S., 1989-2005

OBJECTIVE

To assess influenza vaccination coverage among recommended adult populations in the United States.

METHODS

Data from the 1989 to 2005 National Health Interview Surveys (NHISs), weighted to reflect the civilian, non-institutionalized U.S. population, were analyzed to determine self-reported levels of influenza vaccination among persons aged >or=65 years, persons with high-risk conditions, health care workers (HCW), pregnant women, and persons living in households with at least one identified person at high risk of complications from influenza infection. We stratified data by race/ethnicity to identify racial/ethnic disparities.

RESULTS

Vaccination coverage levels among all recommended adult populations peaked in 2004, then declined in 2005 in association with the 2004-2005 vaccine shortage. Coverage for adults >or=65 years of age increased from 30.1% (95% confidence interval [CI]: 28.8-31.3) in 1989 to 70.0% (68.0-71.5) in 2004. In 2004, coverage was 40.7% (39.0-42.5) for all adults 50-64 years, 27.2% (24.6-29.9) for adults aged 18-49 years with high-risk conditions, 43.2% (39.9-46.6) for health care workers, 21.1% (19.1-23.4) for non-high-risk adults aged 18-64 years with a high-risk household member, and 14.4% (8.8-22.9) for pregnant women. Among each of the recommended adult sub-groups, vaccination coverage was higher for non-Hispanic whites compared to minority groups.

CONCLUSIONS

By 1997, influenza vaccination coverage had exceeded the national 2000 objective of 60% among persons aged >or=65 years, but by 2004 still remains well below the national 2010 target of 90%. Coverage levels for other groups targeted for influenza vaccination also are far short of the Healthy People 2000 and 2010 goals of 60% for persons aged 18-64 years with high-risk conditions, health care workers, and pregnant women. A concerted effort to increase provider adoption of standards for adult immunization, public awareness, and stable vaccine supplies are needed to improve influenza vaccination rates among recommended groups, and to reduce racial and ethnic disparities.

Impact of influenza immunization on medical expenditures among Medicare elderly, 1999-2003

BACKGROUND

The impact of influenza immunization on expenditures for inpatient, outpatient, and professional services among elderly Medicare beneficiaries between 1999 and 2003 was examined.

METHODS

Data were from independent annual survey samples of 175,000 beneficiaries. Response rates ranged from 64% to 71%. Survey data included beneficiaries' demographics, education, supplemental insurance, perceived health, and influenza vaccination. Baseline measures, derived from Medicare claims for the year prior to influenza season, included service utilization, comorbidities, influenza immunization, and health status. The outcome measure was medical expenditures for acute and chronic respiratory conditions (ACRCs) for each 33-week annual influenza season.

RESULTS

Total expenditures for ACRCs were lower among the immunized population during all four seasons. The amount and statistical significance of the savings varied with the severity of the virus and the vaccine match to the prevalent influenza strains. During the 1999-2000 influenza season, which had the most severe virus and a close vaccine match, average costs for ACRCs were $88 lower among immunized beneficiaries than among nonimmunized beneficiaries (equivalent to a 3.06% savings). During the 2002-2003 season, which had a less severe virus but the highest vaccine match rate, average costs for ACRCs were $103 lower for immunized beneficiaries than for nonvaccinated beneficiaries (equivalent to a 3.12% savings). The relative reduction in ACRC expenditures among vaccinated beneficiaries is attributable to less frequent use of inpatient services.

CONCLUSIONS

In addition to improving the health of older Americans, meeting the Healthy People 2010 influenza immunization goal of 90% among the elderly should also result in lower Medicare expenditures.

Effectiveness of the MF59-adjuvanted influenza vaccine in preventing emergency admissions for pneumonia in the elderly over 64 years of age

Case-control study designed to determine the effectiveness of an MF59-adjuvanted influenza vaccine in the population aged 65 years and older living in the community. Detailed health histories were obtained on both cases and controls that included a functional measure of co-morbidity (Barthel Index). Subjects were all eligible persons admitted to various hospitals with a diagnosis of pneumonia during the winter months and were matched by sex, hospital and admission week to controls admitted for non-medical reasons. The influenza vaccination programme using the MF59-adjuvanted influenza vaccine significantly reduced the probability of being hospitalised for pneumonia in the elderly over 64 years of age, even in a season with a low influenza activity, during which the predominant circulating strains were types B and A (H1N1).

Economic evaluation of vaccination against influenza in the elderly: an experience from a population-based influenza vaccination program in Taiwan

Due to viral strains, influenza season, and consultations and admission rates varying from country to country, the continued economic evaluation of influenza vaccination for the elderly people aged 65 years and above is paramount, particularly in areas with dense population. Efficacy and cost-effective analysis of influenza vaccination in reducing all-cause mortality and hospitalization was therefore elucidated based on a prospective and population-based study targeted to 226,997 elderly people aged 65 years and above residing in Taipei county, Taiwan between 1 October 2000 and 31 March 2001. Vaccination against influenza for the elderly persons can lead to a 29% (95% CI: 23-35%) significant reduction of all-cause deaths. Approximately, 20% (95% CI: 9-30%) significant reduction in hospitalization was observed for average-risk group but 4% (95% CI: -4-11%) non-significant reduction for high-risk group. Community-based influenza vaccination program for elderly people aged 65 years and above was demonstrated to be effective in reducing mortality in all elderly people but not significantly in reducing hospitalization. Universal vaccination program for the elderly people seems cost-effective in averting death or gaining life years.

The information network of senior citizens in Geneva, Switzerland, and progress in flu vaccination coverage between 1991 and 2000

Switzerland has lagged behind other industrialized countries in increasing vaccination coverage against flu in the elderly population. The information campaign "United against Flu", started in Geneva in 1993, gradually extended to other French and Italian speaking cantons in Switzerland and indirectly affected German-speaking cantons. Activities developed include the production of TV spots, press conferences, information forwarded to health professionals, an Internet site and information material such as leaflets and posters to risk groups. The campaign is evaluated by repeated surveys that measure vaccination coverage as well as network of informants, knowledge and perceptions in the geriatric population. Vaccination coverage of the geriatric population in Geneva canton has increased from 29% in 1991 to 59% in the year 2000.

Influence of high-risk medical conditions on the effectiveness of influenza vaccination among elderly members of 3 large managed-care organizations

This serial cohort study assessed the risk of hospitalization or death associated with influenza and the effectiveness of influenza vaccination among subgroups of elderly members of 3 managed-care organizations in the United States. Data on baseline characteristics and outcomes were obtained from computerized databases. A total of 122,974 (1996-1997 season) and 158,454 (1997-1998 season) persons were included in the cohorts. Among unvaccinated persons, hospitalizations for pneumonia/influenza or death occurred in 8.2 of 1000 healthy and 38.4 of 1000 high-risk persons in year 1, and in 8.2 of 1000 healthy and 29.3 of 1000 high-risk persons in year 2. After adjustments, vaccination was associated with a 48% reduction in the incidence of hospitalization or death (95% confidence interval [CI], 42-52) in year 1 and 31% (95% CI, 26-37) in year 2. Effectiveness estimates were statistically significant and generally consistent across the healthy and high-risk subgroups. The absolute risk reduction, however, was 2.4- to 4.7-fold higher among high-risk than among healthy elderly persons. All elderly individuals may substantially benefit from vaccination. However, the impact of influenza is greater in persons with high-risk medical conditions.

Medical care capacity for influenza outbreaks, Los Angeles

In December 1997, media reported hospital overcrowding and "the worst [flu epidemic] in the past two decades" in Los Angeles County (LAC). We found that rates of pneumonia and influenza deaths, hospitalizations, and claims were substantially higher for the 1997-98 influenza season than the previous six seasons. Hours of emergency medical services (EMS) diversion (when emergency departments could not receive incoming patients) peaked during the influenza seasons studied; the number of EMS diversion hours per season also increased during the seasons 1993-94 to 1997-98, suggesting a decrease in medical care capacity during influenza seasons. Over the seven influenza seasons studied, the number of licensed beds decreased 12%, while the LAC population increased 5%. Our findings suggest that the capacity of health-care systems to handle patient visits during influenza seasons is diminishing.

Zanamivir: from drug design to the clinic

The development of the neuraminidase inhibitors has revolutionized the management options for influenza. Zanamivir was the first such inhibitor to be approved for the treatment of influenza in humans. It is delivered by inhalation to the respiratory tract, which is the site of viral replication, in order to ensure immediate antiviral activity. Early treatment with zanamivir in clinical trials rapidly reduced the severity and duration of influenza symptoms and associated complications. Furthermore, chemoprophylaxis with zanamivir was shown to be effective in the prevention of influenza illness. To date, there is no evidence for the emergence of clinically significant zanamivir-resistant isolates. In conclusion, zanamivir offers a useful complementary strategy to vaccination in the effective management of influenza.

Influenza vaccination, hospitalizations, and costs among members of a Medicare managed care plan

OBJECTIVE

To evaluate the effectiveness and possible cost savings of influenza vaccination.

SUBJECTS

Members age 65 and older in a Medicare managed care plan during the 1994-1995, 1995-1996, and 1996-1997 influenza seasons.

RESEARCH DESIGN

The study examined administrative data on influenza vaccination and subsequent hospitalizations. Outcomes included hospitalization with pneumonia or influenza, with any respiratory condition, and with congestive heart failure (CHF).

RESULTS

Vaccinated subjects experienced fewer hospitalizations with respiratory conditions or CHF than had unvaccinated subjects (OR=0.8 (95% CI, 0.7, 0.9) in analyses adjusted for age, sex, pneumococcal vaccination, health utilization, and morbidity). Analyses adjusted in addition for ethnicity obtained similar results among the subgroup of members whose ethnicity was known. Subjects without major disease in the previous 12 months had lower odds ratios for vaccination than subjects with major disease (OR values of 0.5 [95% CI, 0.4, 0.7] and 0.9 [95% CI, 0.8, 1.1], respectively). Subjects ages 65 to 79 had lower odds ratios for vaccination than subjects ages 80 and older (OR values of 0.7 [95% CI, 0.6, 0.9] and 0.9 [95% CI, 0.8, 1.1], respectively). Estimated cost savings averaged about $80 per vaccinated subject.

CONCLUSIONS

Subjects ages 65 to 79 who had received influenza vaccination experienced fewer hospitalizations and had lower costs than had unvaccinated subjects. Associations were weaker for subjects age 80 and older. The results, consistent with recommendations for the use of influenza vaccine, suggest that people ages 65 to 79 should be heavily targeted for vaccination.

Influenza vaccines: a review and rationale for use in developed and underdeveloped countries

Multiple studies have demonstrated that influenza infection results in considerable morbidity and mortality, as well as other economic consequences, such as school and work absenteeism. Influenza vaccine has been shown to be both cost-effective and cost-saving. Despite this, the influenza vaccine appears to be under-utilized throughout the world, with significant variations both between countries and within countries over time. Data will be discussed that provide a rationale for the use of the influenza vaccine to protect the public health. Recommendations for the use of the influenza vaccine in various countries and guidelines for influenza vaccine use worldwide will be proposed.

The cost-effectiveness of a hypothetical respiratory syncytial virus vaccine in the elderly

To determine if specific levels of vaccine cost and effectiveness exist that would support eventual respiratory syncytial virus (RSV) vaccine use in the elderly, a cost-effectiveness study was conducted comparing yearly administration of a hypothetical RSV vaccine among the 65-year-old US cohort to medical management of disease. Using base case assumptions - including a vaccine effectiveness against RSV-related hospitalization and death of 80% and a vaccine cost of US$33 - vaccine would result in 0.7 additional days of healthy life at a cost of US$9.82 per person. The cost per quality-adjusted life-year gained equaled US$5342 and remained reasonable over a wide range of vaccine cost and effectiveness. RSV vaccine would be cost-effective for the elderly population, with cost-effectiveness ratios similar to those for influenza vaccine.

Randomized, placebo-controlled double blind study on the efficacy of influenza immunization on absenteeism of health care workers

BACKGROUND

In healthy adults influenza immunization reduces absenteeism caused by respiratory infections, but data on its efficacy among health care workers are scarce.

OBJECTIVE

To determine the effect of the conventional inactivated influenza A vaccine on reducing absenteeism related to respiratory infections among pediatric health care providers.

STUDY DESIGN

A randomized, placebo-controlled, double blind study on vaccine efficacy was conducted in two pediatric hospitals during the winter season 1996 to 1997. The primary endpoint was days of work lost from the hospital because of respiratory infections. The documentation of absenteeism was based on personal sickness logs.

RESULTS

Of the 547 randomized vaccinees 427 (78%) persons completed the 4-month follow-up and returned the sickness logs. Immunization failed to reduce episodes of respiratory infections (1.8 episodes/study period among vaccinees vs. 2.0 among controls). Similarly the vaccine failed to affect the total number of days the vaccinees suffered from respiratory infections (13.5 days vs. 14.6 days, respectively). However, days of work lost because of respiratory infections (1.0 days vs. 1.4 days, respectively, P = 0.02) and especially total numbers of days the study persons felt themselves unable to work when either on or off duty (2.5 days vs. 3.5 days, P 0.02) were significantly decreased.

CONCLUSION

Influenza vaccination reduced absenteeism related to respiratory infections by 28%. We therefore believe that routine annual influenza immunizations should be recommended to health care providers working in pediatric settings.

The economic impact of pandemic influenza in the United States: priorities for intervention

We estimated the possible effects of the next influenza pandemic in the United States and analyzed the economic impact of vaccine-based interventions. Using death rates, hospitalization data, and outpatient visits, we estimated 89,000 to 207,000 deaths; 314,000 to 734,000 hospitalizations; 18 to 42 million outpatient visits; and 20 to 47 million additional illnesses. Patients at high risk (15% of the population) would account for approximately 84% of all deaths. The estimated economic impact would be US$71.3 to $166.5 billion, excluding disruptions to commerce and society. At $21 per vaccinee, we project a net savings to society if persons in all age groups are vaccinated. At $62 per vaccinee and at gross attack rates of 25%, we project net losses if persons not at high risk for complications are vaccinated. Vaccinating 60% of the population would generate the highest economic returns but may not be possible within the time required for vaccine effectiveness, especially if two doses of vaccine are required.

Immunogenicity of trivalent subunit versus virosome-formulated influenza vaccines in geriatric patients

The safety and immunogenicity of a commercial trivalent subunit influenza vaccine and an experimental virosome-formulated influenza vaccine were evaluated among geriatric patients in a double-blind, randomized manner. The virosome vaccine was produced by incorporating hemagglutinin (HA) into the membrane of liposomes composed of phosphatidylcholine. Both vaccines elicited a significant (P < 0.01) rise in the geometric mean anti-HA antibody titer to all three vaccine components 1 month after immunization. However, significantly (P < 0.005) more subjects vaccinated with the virosome preparation mounted a more than fourfold rise to the A/Singapore and A/Beijing strains compared with those who received subunit vaccine. The percentage of patients who attained protective levels (anti-HA titer > or = 40) of anti-A/Beijing antibody was also significantly (P < 0.005) higher in the virosome group. Subjects who possessed non-protective baseline antibody levels to the A/Singapore and A/Beijing strains were more likely (P < 0.005-0.030) to achieve protective levels after immunization with the virosome vaccine than with the subunit vaccine. Of particular clinical significance was the fact that 68.4% of subjects immunized with the virosome vaccine attained protective levels of antibody to all three vaccine components versus 38% for the subunit vaccine (P = 0.010).

The efficacy and cost effectiveness of vaccination against influenza among elderly persons living in the community

BACKGROUND

Despite recommendations for annual vaccination against influenza, more than half of elderly Americans do not receive this vaccine. In a serial cohort study, we assessed the efficacy and cost effectiveness of influenza vaccine administered to older persons living in the community.

METHODS

Using administrative data bases, we studied men and women over 64 years of age who were enrolled in a large health maintenance organization in the Minneapolis-St. Paul area. We examined the rate of vaccination and the occurrence of influenza and its complications in each of three seasons: 1990-1991, 1991-1992, and 1992-1993. Outcomes were adjusted for age, sex, diagnoses indicating a high risk, use of medications, and previous use of health care services.

RESULTS

Each cohort included more than 25,000 persons 65 years of age or older. Immunization rates ranged from 45 percent to 58 percent. Although the vaccine recipients had more coexisting illnesses at base line than those who did not receive the vaccine, during each influenza season vaccination was associated with a reduction in the rate of hospitalization for pneumonia and influenza (by 48 to 57 percent, P < or = 0.002) and for all acute and chronic respiratory conditions (by 27 to 39 percent, P < or = 0.01). Vaccination was also associated with a 37 percent reduction (P = 0.04) in the rate of hospitalization for congestive heart failure during the 1991-1992 season, when influenza A was epidemic. The costs of hospitalization for all types of illness studied were lower in the vaccinated group during 1991-1992 (range of reduction, 47 to 66 percent; P < 0.005) and for acute and chronic respiratory conditions and congestive heart failure in 1990-1991 (reductions of 37 percent and 43 percent, respectively; P < or = 0.05). Direct savings per year averaged $117 per person vaccinated (range, $21 to $235), with cumulative savings of nearly $5 million. Vaccination was also associated with reductions of 39 to 54 percent in mortality from all causes during the three influenza seasons (P < 0.001).

CONCLUSIONS

For elderly citizens living in the community, vaccination against influenza is associated with reductions in the rate of hospitalization and in deaths from influenza and its complications, as compared with the rates in unvaccinated elderly persons, and vaccination produces direct dollar savings.