Physician Attitudes and Preferences About Combined
Tdap Vaccines for Adolescents
Matthew M. Davis, MD, MAPP, Karen R. Broder, MD, Anne E. Cowan, MPH, Christina Mijalski, MPH,
Katrina Kretsinger, MD, MA, Shannon Stokley, MPH, Sarah J. Clark, MPH
Combined tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis vaccine
(Tdap) boosters for adolescents are a new strategy to prevent pertussis. We examined the
current practices of pediatricians and family physicians regarding adolescent tetanus and
diphtheria toxoids (Td) vaccine immunizations and providers’ potential adherence to new
Tdap recommendations for adolescents.
Using a brief survey instrument sent to a random sample of pediatricians and family
physicians in January 2005, we assessed providers’ patterns of administration of Td
boosters, barriers to Td boosters, and agreement that pertussis vaccination of adolescents
is warranted. Results of analyses in February 2005 were presented to the Advisory
Committee on Immunization Practices of the Centers for Disease Control and Prevention
(CDC) to inform its deliberations regarding adolescent Tdap vaccination.
The overall response rate was 56% (57% pediatricians, 55% family physicians). Among 297
respondents (154 pediatricians, 143 family physicians) eligible for analysis because they
provide care to adolescents, pediatricians (77%) were significantly more likely than family
physicians (51%, p ?0.0001) to report that they routinely administer Td at preventive care
visits for adolescents aged 11 to 12 years, but otherwise the specialties were similar in their
Td practices. Forty-four percent of respondents cited infrequency of adolescent visits as a
barrier to Td immunization. Slightly more than half the sample (57%) agreed or strongly
agreed that pertussis is serious enough to warrant replacing Td with Tdap for adolescents;
pediatricians (70%) were significantly more likely than family physicians (42%, p ?0.0001)
to endorse this statement.
This national survey indicates moderate willingness, stronger among pediatricians than
among family physicians, to support recommendations for Tdap among adolescents. In
February 2006, CDC released recommendations that adolescents aged 11 to 18 (preferred
age 11 to 12) receive a single dose of Tdap in place of Td if they have not already received
the latter. Near-term efforts regarding Tdap recommendations must address providers’
concerns about infrequent routine visits for adolescents and convince more physicians of
the importance of pertussis booster immunization during adolescence.
(Am J Prev Med 2006;31(2):176–180) © 2006 American Journal of Preventive Medicine
sis remains endemic in the United States (25,827 cases
reported in 2004),2and incidence rates of reported
among children aged 19 to 35 months reached
an all-time high in 2004.1Nevertheless, pertus-
vaccinationrates against pertussis
pertussis in the United States have climbed steadily over
the past 4 decades, especially among adolescents and
adults whose immunity from childhood vaccination has
Providers frequently do not recognize respiratory
symptoms in adolescents and adults as signs of pertussis
illness,4yet the health and economic consequences of
pertussis in adolescents can be substantial.5Vaccinating
adolescents against pertussis is an appealing strategy to
prevent pertussis in this age group, and immunization
programs in Australia, Austria, Canada, France, and
Germany have recently added adolescent pertussis
boosters to their immunization schedules.6,7Two re-
cently licensed tetanus toxoid, reduced diphtheria tox-
oid, and acellular pertussis vaccine (Tdap) products for
adolescents have been shown to induce adequate im-
From the Child Health Evaluation and Research Unit, Division of
General Pediatrics (Davis, Cowan, Clark), Division of General Inter-
nal Medicine (Davis), Gerald R. Ford School of Public Policy (Davis),
University of Michigan, Ann Arbor, Michigan; National Immuniza-
tion Program, Centers for Disease Control and Prevention (Broder,
Mijalski, Kretsinger, Stokely), Atlanta, Georgia; and Commissioned
Corps of the United States Public Health Service (Broder, Kretsinger),
Address correspondence and reprint requests to: Matthew M.
Davis, MD, MAPP, University of Michigan, 300 NIB, 6C23, Ann Arbor
MI 48109-0456. E-mail: email@example.com.
Am J Prev Med 2006;31(2)
© 2006 American Journal of Preventive Medicine • Published by Elsevier Inc.
0749-3797/06/$–see front matter
mune responses to all vaccine antigens, with an overall
safety profile similar to that of tetanus and diphtheria
toxoids vaccine (Td).8–10Administering a Tdap booster
to adolescents has been estimated to cost $20,000 per
quality-adjusted life-year saved.11
In the United States, adolescents visit pediatricians
and family physicians in comparable proportions in the
outpatient setting.12Adolescent Td vaccination prac-
tices have not been well defined for either group of
physicians, and Td coverage estimates have varied
widely.13In addition, several studies have indicated that
pediatricians more readily adopt new national vaccine
recommendations than do family physicians.14–17Such
studies were conducted regarding vaccinations admin-
istered in early childhood, however, not in adolescence.
In previous studies, pediatricians and family physicians
agree that barriers to adolescent vaccination, such as
infrequent preventive care visits and lower rates of
insurance coverage compared to younger children,
pose specific challenges to successful immunization
efforts in this age group.18–20
A survey was conducted of pediatricians and family
physicians in order to characterize their current Td
booster vaccination practices, perceived barriers to Td
vaccination, attitudes regarding pertussis, and their
preferences regarding a Tdap recommendation for
adolescents. Findings were presented to the Advisory
Committee on Immunization Practices (ACIP) of the
Centers for Disease Control and Prevention (CDC) in
February 2005 to inform its deliberations regarding
adolescent Tdap vaccination. In June 2005, the ACIP
voted to recommend Tdap universally for adolescents;
CDC issued final ACIP recommendations in February
A national random sample of 298 pediatricians and 296
family physicians was drawn from the American Medical
Association (AMA) Masterfile, a database of all licensed U.S.
physicians, through a contracted vendor (Medical Marketing
Services). The sampling frame included all allopathic and
osteopathic physicians self-described as a general pediatrician
or family physician, in office-based direct patient care. Ex-
cluded were physicians with any subspecialty board listing,
physicians aged ?70 years, resident physicians, and physicians
practicing at Veterans Administration facilities. The study was
approved by the institutional review boards of the University
of Michigan Medical School and the CDC.
The study team developed a one-page, seven-item survey
instrument, accompanied by a one-page “fact sheet” re-
garding pertussis and Tdap vaccines in development at the
time of the survey. The fact sheet presented information
about pertussis epidemiology and unlicensed Tdap vac-
cines that were under review by the Food and Drug
Administration; it is available online at the American Journal
of Preventive Medicine website (www.ajpm-online.net). Sur-
vey items included the volume of adolescent patients,
patterns of administration of Td boosters, barriers to Td
boosters, enforcement of school requirements for Td
boosters, extent of agreement that pertussis vaccination of
adolescents is warranted, preferences for future adolescent
vaccine recommendations, and practice setting.
The survey instrument was pilot tested with a convenience
sample of primary care pediatricians and family physicians to
ensure clarity and ease of administration. Refinements were
made based on pilot-test feedback.
In order to meet the timeframe of the ACIP, only one
mailing of the survey was fielded, in January 2005. Survey
packets contained a cover letter explaining the purpose of
the study, the fact sheet, survey form, and $5 cash
Initial univariate frequencies were generated for each
variable. Chi-square analyses were performed to explore
associations between variables. A two-tailed alpha level of
0.05 was used as the threshold for statistical significance.
All analyses were conducted in 2005 using SAS, version 8.2
(SAS Institute, Cary NC, 2001).
Of 594 physicians in the study sample, 19 were
excluded (9 pediatricians, 10 family physicians) be-
cause mailing materials were returned as undeliver-
able. Surveys were returned by 321 respondents (165
pediatricians, 156 family physicians), for an overall
response rate of 56% (57% pediatricians, 55% family
physicians). A total of 24 respondents (11 pediatri-
cians, 13 family physicians) were ineligible because
they indicated that they were not providing outpa-
tient primary care to adolescents, leaving 297 respon-
dents (154 pediatricians, 143 family physicians) in
the group for final analysis. Of these, 71% were in
private practice, 13% in hospital-based practice, 9%
in practice networks (including managed care orga-
nizations), and the remainder in other settings.
Based on available data from the original sample
frame, respondents did not differ from nonrespon-
dents with regard to degree (medical doctor vs
doctor of osteopathy), specialty (family practice vs
pediatrics), age, gender, years since medical school
graduation, or board certification status.
Current Td Immunization Patterns
Pediatricians were significantly more likely than fam-
ily physicians to report seeing more than ten adoles-
August 2006 Am J Prev Med 2006;31(2)
cent patients per week (Table 1). Pediatricians and
family physicians reported similar Td immunization
practices regarding wound management, and also
reported similar rates of routine Td immunization at
routine preventive care visits for adolescents aged 13
to 18 years. However, pediatricians were significantly
more likely than family physicians to report that they
routinely administer Td at preventive care visits for
adolescents aged 11 to 12 years.
Physicians endorsed barriers at the community and
individual level. Overall, 44% of respondents cited
the infrequency of adolescent visits as a barrier to Td
immunization, but this barrier did not differ by
specialty. Forty percent of respondents reported no
barriers to Td immunization, but the proportion
differed by specialty (pediatricians 47% vs family
physicians 32%, p ?0.01). Accordingly, family physi-
cians were more likely than pediatricians to report
the following barriers to Td administration: inade-
quate reimbursement (17% vs 3%, p ?0.0001), inad-
equate immunization record-keeping (24% vs 11%,
p ?0.005), and being too busy in practice (7% vs 2%,
Slightly over half (54%) of the sample reported
that schools in their communities enforce a middle-
or high-school requirement for Td boosters, 25%
reported no enforcement, 17% were unsure, and 4%
said that the question was not applicable to their
community. Family physicians were significantly
more likely than pediatricians to indicate that they
were unsure about Td school mandates (22% vs 13%,
Provider Attitudes Regarding Potential
Slightly more than half the sample (57%) agreed or
strongly agreed with the statement that “pertussis is a
serious enough disease to warrant administering a
vaccine that includes an acellular pertussis component
(Tdap), rather than Td, for adolescents.” As shown in
Figure 1, pediatricians were significantly more likely
than family physicians to endorse this statement, with
70% of pediatricians indicating that they agreed or
strongly agreed versus only 42% of family physicians
(p ?0.0001). Few physicians disagreed or strongly dis-
agreed with this statement, and the remainder—includ-
ing more than one of every three family physicians—
Pediatricians and family physicians were in broad
agreement about how new adolescent vaccine recom-
mendations should be structured, with 60% favoring
the consolidation of recommendations at a single age
and 40% favoring recommendations targeted to spe-
cific ages based on incidence of disease.
In February 2006, the CDC released final ACIP recom-
mendations that adolescents aged 11 to 18 receive a
single dose of Tdap in place of Td if they have not
already received the latter (the preferred age for Tdap
vaccination is 11 to 12 years).21Results of this national
survey, which informed the recommendations, indicate
Figure 1. Physician attitudes regarding future Tdap recom-
mendations. Response to the statement: “Pertussis is a serious
enough disease to warrant administering a vaccine that
includes an acellular pertussis component (Tdap), rather
than Td, for adolescents.”
Table 1. Respondents’ practice characteristics regarding
adolescent visits and Td booster dose practices
Number of adolescent patients per week
Administer Td at visit for wound managementa
Administer Td at visit for routine care, age 11–12 yearsa
Administer Td at visit for routine care, age 13–18 yearsa
NS, not significant; Td, tetanus/diphtheria.
American Journal of Preventive Medicine, Volume 31, Number 2www.ajpm-online.net
a moderate willingness among physicians to support
replacement of the adolescent Td booster with Tdap.
Most of the surveyed physicians reported routinely
administering Td to adolescents for wound care and
during routine preventive care visits when indicated.
However, pediatricians were more likely than family
physicians to report using Td routinely at the 11- to
12-year-old preventive visit, which has been the pre-
ferred age for Td administration since 1995.22As-
suming that physicians use Tdap in a similar manner
to Td, pediatricians might be more likely than family
physicians to vaccinate younger adolescents with
Tdap to reduce morbidity associated with pertussis in
These findings present two central challenges in
interpreting the clinical implications of this study. First,
this survey was an effort to measure providers’ potential
adoption of adolescent Tdap recommendations, rather
than responses to an existing recommendation. The
extent to which providers’ current adolescent Td vac-
cination practices will predict their Tdap vaccination
practices is unknown. Second, it cannot be said with
certainty how the proportions of providers who agree,
in this survey, that adolescent Tdap vaccination is
warranted will correspond to post-recommendation
adoption rates. However, disparities between pediatri-
cians and family physicians in potential support for
Tdap are similar to those reported in post-recommen-
dation surveys for several other vaccines,14–17which
suggests that these pre-recommendation findings may
offer an informative preview of post-recommendation
Differences in Td vaccination practices and current
support for Tdap among pediatricians and family phy-
sicians underscore the importance of visit patterns for
adolescents to these providers for future vaccination
efforts. A recent analysis of national visit trends for
adolescents indicates that comparable proportions of
adolescent encounters occur in pediatric and family
medicine settings, a balance that contrasts starkly with
the predominance of pediatric settings for infant vis-
its.12In the current sample, pediatricians reported
higher volumes of adolescent patients than family
physicians on average, but this may be offset at the
national level by greater numbers of office-based family
physicians than pediatricians. Therefore, whereas pedi-
atricians’ typically stronger support for vaccinations
may drive high pertussis (diphtheria and tetanus tox-
oids and acellular pertussis vaccine [DTaP]) coverage
rates in early childhood, family physicians’ lower enthu-
siasm for Tdap measured in this study may pose a
barrier to the success of a national adolescent Tdap
campaign. Potential remedies for this challenge in-
clude attempts to educate family physicians and prompt
greater concern about the morbidity of pertussis in
adolescents, particularly if family physicians are gener-
ally less aware of pertussis morbidity because of fewer
infant visits. Other initiatives that address perceived
barriers to adolescent vaccination, emphasize efforts to
identify missed opportunities to vaccinate adolescents
(e.g., at acute care visits and sports physicals), and
induce public demand for Tdap regardless of provid-
ers’ attitudes may be helpful.
As a group, adolescents are the least likely pediatric
population to have routine preventive care visits, and
therefore many providers agree that adolescent vac-
cines may need to be administered across a variety of
settings including schools and teen clinics.18,19Never-
theless, challenges of insurance reimbursement for
vaccines administered at school, in addition to prob-
lems ensuring medical record continuity between
school and medical home settings, may pose formida-
ble obstacles to the use of schools and other community
venues as alternative immunization sites. Even at doc-
tors’ offices, the steadily rising costs of the recom-
mended childhood immunization series23and some
private health plans’ reluctance to cover the costs of
newer vaccines may also present barriers to the rapid
adoption of Tdap recommendations by physicians and
Studies utilizing mailed surveys have inherent limitations,
chiefly the possibility of response bias. It is possible that
those who responded to the survey were more interested
in vaccination issues; at the same time, it is likely that
they do not as actively endorse childhood immunizations.
The potential for response bias exists, but it is impossible
to detect its direction. Although differences in attitudes
between respondents and nonrespondents could not be
assessed, there were no differences in the practice and
demographic data available regarding the two groups.
Furthermore, the interpretation of these findings is bol-
stered by the pediatrician versus family physician differ-
ences that are consistent with post-recommendation stud-
ies, as well as with specialty differences in a recent pre-
recommendation study of early childhood influenza
In addition, the overall response rate for this na-
tional, time-sensitive, one-wave survey is consistent with
those of other published studies of physician behavior,
suggesting that the study sample was neither more nor
less likely to answer questions about future immuniza-
tion practices than about other issues.28–30
These study findings suggest that most physicians will
accept the February 2006 ACIP recommendations to
routinely vaccinate adolescents with Tdap instead of
Td. However, securing sufficient physician support,
especially among family physicians, to ensure a rapid
August 2006Am J Prev Med 2006;31(2)
uptake of the Tdap recommendation for adolescents
aged 11 to 12 years and a high Tdap coverage of
adolescents may require additional efforts to educate
and motivate physicians. It is not yet clear whether the
high level of adherence to the recommendation for
infant immunization against pertussis can be replicated
for adolescent care. Therefore, near-term efforts in
support of a Tdap recommendation must first focus on
convincing physicians who are not yet persuaded of the
importance of pertussis booster immunization during
This work was funded by the Centers for Disease Control and
Prevention. We recognize the ACIP Pertussis Working Group
and CDC Pertussis Team for their contributions to this study.
The findings and conclusions in this report are those of the
authors and do not necessarily represent the views of the
No financial conflict of interest was reported by the authors
of this paper.
1. Centers for Disease Control and Prevention. National, state, and urban
area vaccination coverage among children aged 19–35 months—United
States, 2004. MMWR Morb Mortal Wkly Rep 2005;54:717–21.
2. Centers for Disease Control and Prevention. Final 2004 reports of notifi-
able diseases. MMWR Morb Mortal Wkly Rep 2005;54:770.
3. Guris D, Strebel PM, Bardenheier B, et al. Changing epidemiology of
pertussis in the United States: increasing reported incidence among
adolescents and adults, 1990–1996. Clin Infect Dis 1999;28:1230–7.
4. Nennig ME, Shinefield HR, Edwards KM, Black SB, Fireman BH. Preva-
lence and incidence of adult pertussis in an urban population. JAMA
5. Lee GM, Lett S, Schauer S, et al. Societal costs and morbidity of pertussis
in adolescents and adults. Clin Infect Dis 2004;39:1572–80.
6. Halperin SA. Canadian experience with implementation of an acellular
pertussis vaccine booster–dose program in adolescents: implications for
the United States. Pediatr Infect Dis J 2005;24(suppl 6):S141–6.
7. Wirsing von Konig C-H, Campins-Marti M, Finn A, Guiso N, Mertsola J,
Liese J. Pertussis immunization in the global pertussis initiative European
region: recommended strategies and implementation considerations. Pe-
diatr Infect Dis J 2005;24(suppl 6):S87–92.
8. Pichichero MB, Rennels MB, Edwards KM, et al. Combined tetanus,
diphtheria, and 5-component pertussis vaccine for use in adolescents and
adults. JAMA 2005;early release. Available at: http://jama.ama-assn.org/
cgi/content/full/293.24.3003v1. Accessed June 9, 2005.
9. U.S. Food and Drug Administration. Boostrix®(tetanus toxoid, reduced
diphtheria toxoid and acellular pertussis vaccine, adsorbed). GlaxoSmith-
Kline (GSK) Biologicals, May 2005. Available at: www.fda.gov/cber/label/
tdapgla050305LB.pdf. Accessed July 17, 2005.
10. U.S. Food and Drug Administration. Tetanus toxoid, reduced diphtheria
toxoid, and acellular pertussis vaccine adsorbed ADACEL?. Aventis Pas-
teur Limited,June 2005. Available
tdapave061005LB.pdf. Accessed July 17, 2005.
11. Lee GM, LeBaron C, Murphy TV, Lett S, Schauer S, Lieu TA. Pertussis in
adolescents and adults: should we vaccinate? Pediatrics 2005;115:1675–84.
12. Freed GL, Nahra T, Wheeler JRC. Which physicians are providing health
care to America’s children? Trends and changes over the last 20 years. Arch
Pediatr Adol Med 2004;158:22–6.
13. Centers for Disease Control and Prevention. Record of the meeting of the
Advisory Committee on Immunization Practices, February 10–11, 2005.
Available at: www.cdc.gov/nip/ACIP/minutes/acip-min-feb05.pdf. Ac-
cessed July 17, 2005.
14. Freed GL, Freeman VA, Clark SJ, Konrad TR, Pathman DE. Pediatrician
and family physician agreement with and adoption of universal hepatitis B
immunization. J Fam Pract 1996;42:587–92.
15. Schaffer SJ, Bruno S. Varicella immunization practices and the factors that
influence them. Arch Pediatr Adolesc Med 1999;153:357–62.
16. Kimmel S, Puczynski S, McCoy RC, Puczynski MS. Practices of family
physicians and pediatricians in administering poliovirus vaccine. J Fam
17. Davis MM, Ndiaye SM, Freed GL, Clark SJ. One-year uptake of pneumo-
coccal conjugate vaccine: a national survey of family physicians and
pediatricians. J Am Board Fam Pract 2003;16:363–71.
18. Humiston SG, Rosenthal SL. Challenges to vaccinating adolescents: vaccine
implementation issues. Pediatr Infect Dis J 2005;24(suppl 6):S134–40.
19. Schaffer SJ, Humiston SG, Shone LP, Averhoff FM, Szilagyi PG. Adolescent
immunization practices: a national survey of U.S. physicians. Arch Pediatr
Adolesc Med 2001;155:566–71.
20. Oster NV, McPhillips-Tangum CA, Averhoff F, Howell K. Barriers to
adolescent immunization: a survey of family physicians and pediatricians.
J Am Board Fam Pract 2005;18:13–9.
21. Centers for Disease Control and Prevention. Preventing tetanus, diphthe-
ria, and pertussis among adolescents: use of tetanus toxoid, reduced
diphtheria toxoid, and acellular pertussis vaccines. MMWR Morb Mortal
Wkly Rep 2006;55:1–43.
22. Centers for Disease Control and Prevention. Recommended childhood
immunization schedule—United States, 1995. MMWR Morb Mortal Wkly
23. Davis MM, Zimmerman JL, Wheeler JRC, Freed GL. The costs of childhood
vaccine purchase in the public sector: past trends, future expectations.
Am J Public Health 2002;92:1982–7.
24. Davis MM, Andreae M, Freed GL. Physicians’ early challenges related to the
pneumococcal conjugate vaccine. Ambul Pediatr 2001;1:302–5.
25. Davis MM, Ndiaye SM, Freed GL, Kim CS, Clark SJ. Influence of insurance
status and vaccine cost on physicians’ administration of pneumococcal
conjugate vaccine. Pediatrics 2003;112:521–6.
26. Institute of Medicine. Financing vaccines for the 21st century. Washington
DC: National Academies Press, 2003.
27. Humiston SG, Szilagyi PG, Iwane MK, et al. The feasibility of universal
influenza vaccination for infants and toddlers. Arch Pediatr Adolesc Med
28. Asch DA, Jedriewski K, Christakis DA. Response rates to mailed surveys
published in medical journals. J Clin Epidemiol 1997;50:1129–36.
29. Cummings SM, Savitz LA, Konrad TR. Reported response rates to mailed
physician questionnaires. Health Serv Res 2001;35:1347–55.
30. Cull WL, O’Connor KG, Sharp S, Tang S-S. Response rates and response
bias for 50 surveys of pediatricians. Health Serv Res 2005;40:213–26.
American Journal of Preventive Medicine, Volume 31, Number 2www.ajpm-online.net
FACT SHEET on the Upcoming Tdap Vaccine Recommendation Download full-text
The Advisory Committee on Immunization Practices (ACIP) is expected to discuss a future
recommendation about a new tetanus-diphtheria-pertussis combination vaccine (Tdap) at its February
2005 meeting. The ACIP seeks to balance scientific issues of disease incidence and vaccine
effectiveness with practical issues related to implementation of the recommendations. To do so, the
ACIP needs to understand current practices for the administration of adolescent Td boosters, as
well as physicians’ preferences regarding a new recommendation.
Responses to the enclosed survey will assist the ACIP in its deliberations. The University of
Michigan is collaborating with the CDC on this study, to ensure that the perspectives of vaccine
providers are considered appropriately. Background information on pertussis disease and the new
Tdap vaccine is provided below.
Pertussis disease. Although 85% of US children receive a primary series of 3 doses of DTaP and at
least one booster dose, pertussis continues to cause substantial disease. In 2003, a total of 11,647 cases
of pertussis were reported to CDC—the highest number reported since 1964. Of these, 4,540 occurred
in adolescents (see Figure below). Moreover, the true number of cases is unknown, as pertussis often
Adolescents and adults become susceptible to pertussis when immunity wanes, approximately 5-10
years after childhood vaccination. Adolescents and adults can transmit pertussis to infants who have
not completed the primary vaccine series, and can perpetuate pertussis disease in the community.
Currently, no pertussis vaccines are licensed in the United States for use in persons over 6 years of age.
Figure: Age Distribution of Reported Pertussis Cases, United States, 2003
<6m 6-11m 1-45-910-14 15-1920-29 30-3940-49 50+
Age group (yrs)
Number of cases
Incidence per 100,000
Number of casesIncidence rate
New Tdap vaccines. In the summer of 2004, two manufacturers submitted licensure applications to
the FDA for adolescent/adult pertussis booster vaccines (Tetanus Toxoid, Reduced Diphtheria Toxoid
and Acellular Pertussis Vaccine Adsorbed), referred to as Tdap. The licensing indication for one
vaccine is for persons 10-18 years of age, while the other is for persons 11-64 years of age.
Adolescents may not be eligible to receive Tdap within 5 years of receiving the Td booster. Costs for
the new vaccines are not yet known.
Am J Prev Med 2006;31(2)