Effectiveness
of Influenza Immunization Postcard Reminders Among Seniors Vaccinated
the Previous Year |
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by Allison
E. Clayton, MPH; Heather Homestead
Introduction
Each Fall since 1993, the Kaiser Permanente Northeast
Division (KPNE) has launched a comprehensive awareness campaign
to promote influenza vaccination among senior members who are
at high risk for complications of influenza. Approximately six
percent of the Division's membership is aged 65 years or older
(N = 32,875 as of November 1997). The campaign is targeted to
members and practitioners, and includes a postcard reminder, informational
articles in member, staff, and practitioner newsletters, promotional
posters, educational brochures, and patient lists for practitioner
follow-up to ensure vaccination. The production and postage costs
for the postcard reminder represent 85% of the annual outreach
campaign expenses.
The annual random telephone survey assesses the vaccination
coverage level among KPNE members aged 65 years and older, and identifies
barriers to and predictors of vaccination for this population. Bivariate
and logistic regression analyses of the results have consistently
confirmed that the single most important predictor of one's likelihood
of receiving vaccination is a positive history of receiving vaccination
the previous year (P<0.001).
This study examines the independent effectiveness
of the postcard reminder intervention among KPNE members aged 65
years or older. The specific hypotheses investigated are the dominant
role of habit in predicting vaccination, and the relative unimportance
of the postcard reminder. Research of this nature allows us to identify
programs with little or no impact and to reallocate resources from
these programs to other programs which are beneficial.
Methods
The study population consisted of 5,278 KPNE members
aged 65 years or older who had been vaccinated against influenza
the previous year (Fall 1996) according to administrative data (Table
1). The entire study population received the standard member education
materials, and all practitioners received the same support information.
Half of the study population (n = 2,631) were randomly selected
to receive the postcard intervention in addition to the standard
member education materials. The two study groups were followed for
three months (October-December 1997), and vaccination coverage levels
were assessed each month.
Any statistically significant difference in vaccination coverage
rates was evaluated with the normal approximation to a binomial
test for differences in two proportions at a conservative significance
level (a = 0.1). The relative risk (RR) was estimated to measure
the association between history of vaccination in 1996 and likelihood
of receiving vaccination in 1997. The chi-square test for measuring
association was calculated to evaluate the degree of association
between history of vaccination and likelihood of vaccination. Confidence
intervals were computed for all statistics at a significance level
of a = 0.05. An analysis of the financial implications of the proposed
strategy was conducted for KPNE and was extrapolated to all Kaiser
Permanente Divisions nationally.
Results
The vaccination proportions among the intervention and
control groups were statistically indistinguishable (78.6% and 77.2%
respectively, Fig. 1, P>0.1). The large sample size allowed for
greater than 99% power to detect a clinically meaningful 5% difference
in vaccination coverage between the two study groups, were one to
exist. In addition, all members were vaccinated at approximately
the same pace in 1997 regardless of vaccination history and postcard
intervention status, with more than 75% of all vaccinations administered
by October 31, 1997 (Fig. 2). Finally, those with evidence of vaccination
in 1996 were more than twice as likely to receive vaccination in
1997 than those without history of vaccination (77.9% and 32.0%
respectively, RR = 2.43, Fig. 3, P<0.001).
Conclusions
The postcard reminder intervention did not affect the
likelihood of vaccination among those who had been immunized the
previous year. This finding substantiates the hypothesis that habit
is the defining predictor of the probability of vaccination. Emphasis
should be placed on intensive outreach to individuals at highest
risk for not receiving vaccination. Those at highest risk are those
who did not receive vaccination the previous year and those who
have not been vaccinated by the end of October. Strategically directing
the postcards to those who were not vaccinated the previous year
will reduce costs by 20% and will allow reallocation of resources
to more intensive outreach among those not vaccinated by the end
of October.
This study demonstrates the defining role of habit
in predicting the likelihood of vaccination among seniors. Those
who seek vaccination do so routinely, and those who do not are just
as habitual in their abstention, with very little migration between
these two behavior patterns. Recognizing the importance of the first
flu shot in establishing a habit of vaccination, further research
will be conducted to identify predictors of first-time immunization
as well as predictors of repeated abstention from immunization.
These findings will catalyze a focused effort to recruit and retain
those who have never been vaccinated. In addition, such a study
will likely inform other preventive health programs such as mammography
and colorectal cancer screening.
For the complete manuscript of this
article, please contact Allison Clayton, MPH, at One CHP Plaza,
Latham, NY 12110.
