To determine the reasons adolescents are not vaccinated for specific vaccines and how these reasons have changed over time.
We analyzed the 2008–2010 National Immunization Survey of Teens examining reasons parents do not have their teens immunized. Parents whose teens were not up to date (Not-UTD) for Tdap/Td and MCV4 were asked the main reason they were not vaccinated. Parents of female teens Not-UTD for human papillomavirus vaccine (HPV) were asked their intent to give HPV, and those unlikely to get HPV were asked the main reason why not.
The most frequent reasons for not vaccinating were the same for Tdap/Td and MCV4, including “Not recommended” and “Not needed or not necessary.” For HPV, the most frequent reasons included those for the other vaccines as well as 4 others, including “Not sexually active” and “Safety concerns/Side effects.” “Safety concerns/Side effects” increased from 4.5% in 2008 to 7.7% in 2009 to 16.4% in 2010 and, in 2010, approaching the most common reason “Not Needed or Not Necessary” at 17.4% (95% CI: 15.7–19.1). Although parents report that health care professionals increasingly recommend all vaccines, including HPV, the intent to not vaccinate for HPV increased from 39.8% in 2008 to 43.9% in 2010 (OR for trend 1.08, 95% CI: 1.04–1.13).
Despite doctors increasingly recommending adolescent vaccines, parents increasingly intend not to vaccinate female teens with HPV. The concern about safety of HPV grew with each year. Addressing specific and growing parental concerns about HPV will require different considerations than those for the other vaccines.
Comments
Parental Intentions for HPV Vaccination of their Daughters
We read the article, "Reasons for not Vaccinating Adolescents: National Immunization Survey of Teens, 2008-2010," by Darden, et.al. with interest. The authors assess adolescent vaccination status with the human papillomavirus (HPV) vaccine, clinician recommendations for the vaccine, parental intent to receive the vaccine, and reasons for not receiving the vaccine. This study concludes that "despite doctors increasingly recommending adolescent vaccines, parents increasingly intend not to vaccinate female teens with HPV." We reach different conclusions based on our analysis and interpretation of the same data. Parental responses to the question regarding intent to vaccinate their daughters against HPV were included in the 2010 public-use file (PUF), although they were excluded in the 2008 and 2009 PUF. In order to assess parental intent for HPV vaccination within the next twelve months from 2008-2010, the authors report the proportion of parents that responded to the question asking main reasons for not intending to receive HPV vaccination as a proxy since only those who reported having <3 HPV doses and responded "not too likely" and "not likely at all" were asked the main reason question. However, those who responded "not sure/don't know" to the vaccine intention question 9.0% [Confidence Interval [CI]=8.2 -10.0] in 2008; 7.2% [CI=6.5-7.9] in 2009; 6.9% [CI=6.1-7.8] in 2010) also received the main reasons question. The use of responses to this question as an indicator of non-intent to receive vaccination misclassifies those who were unsure or didn't know of their intentions as those with non- intent for vaccination, which can lead to the mis-interpretation of the findings. A greater concern is that the analysis mixed two distinct populations, those who were reported by the parent to have received zero HPV doses and those reported to have received one or two doses. We analyzed 2008-2010 NIS-Teen data to assess trends in non-intention to receive HPV vaccination (i.e., not too likely, not likely at all, not sure/don't know) among the total number of females in the survey and stratified by parent-reported vaccination status. The percent of unvaccinated females with non- intentions for vaccination remained stable from 36.2% (CI=34.8-37.7) in 2008 to 37.4% (CI=36.0-38.8) in 2010. However, the number of females who started the series and with non-intentions for further vaccination increased from 2.5% (CI=2.1-2.9) in 2008 to 5.1% (CI=4.6-5.7) in 2010. We theorize that the increase in non-intentions among those who started the series may be partly due to parents mistakenly believing that their daughter already received all of their shots. The authors' conclusions are overstated when not accounting for differences between unvaccinated females and those who started the HPV series. Non-intentions for vaccination did not change among the unvaccinated over time, yet increased among those who started the series. In 2010, only 5% of girls who had started but not completed the series had non-intentions for further doses. Although we do not agree with the authors conclusions regarding HPV vaccine intentions, we agree that national-level coverage with the HPV vaccine is low and efforts to increase initiation and completion of the series are needed.
Conflict of Interest:
None declared
The importance of considering changes in the eligible population over time when interpreting human papillomavirus vaccination trends
We read with interest the article by Darden et al. [1], in which the authors explored time-trends (2008 - 2010) in up-to-date (i.e. completed the full 3-dose course) vaccination status for adolescents in the United States and the reasons for not seeking vaccination. One finding that has already received considerable media coverage was an observed increase in the proportion of parents who indicated that they did not intend to have their adolescent daughters vaccinated for HPV. We caution against overemphasizing this finding of decreasing intent to vaccinate, and illustrate why changes in the eligible population during the study period are important to consider when interpreting this result.
For simplicity, assume that we randomly selected 100 adolescent females (i.e. a sample of the target population) for whom we ascertained up-to-date HPV vaccination status in 2008 and 2010. We observed that up-to -date HPV vaccine coverage for this population was 18% (18/100) in 2008 and 32% (32/100) in 2010, consistent with estimates from the National Immunization Survey-Teen (NIS-Teen) for those years [2, 3]. These percentages imply that 82 adolescent females were not up-to-date for the HPV vaccine in 2008 and 68 adolescent females were not up-to-date in 2010. Darden et al. [1] reported that among adolescent females who were not up- to-date, the percentage of parents who did not intend to have their daughters vaccinated for HPV within the next year increased from 39.8% in 2008 to 43.9% in 2010. Applied to our random sample, the number of parents who did not intend to have their daughters vaccinated for HPV within the next year would equate to 32.6 (82*0.398) in 2008 and 29.9 (68*0.439) in 2010. These numbers convert to 32.6% in 2008 and 29.9% in 2010 in the context of our full sample of 100 adolescent females, accounting for the relative percentage of adolescent females already vaccinated for HPV.
Our example illustrates that as more adolescent females become up-to- date for the HPV vaccine over time, the proportion of parents who indicate no intent to vaccinate their adolescent daughters could decrease in the context of the overall population despite the apparent increase reported by Darden et al. [1]. In essence, the subset of parents who indicate no intent to vaccinate for HPV gradually comprise a larger relative proportion of a declining population of females eligible for up-to-date vaccination. In a longitudinal cohort, this phenomenon is attributable to changes in the population at risk over time as individuals have the event of interest or are censored [4]. In a serial cross-sectional survey of independent samples, such as the NIS-Teen data used in the Darden et al. [1] study, this change in the relative proportion of eligible subjects over time cannot be easily distinguished, and may lead to inaccurate results. Consequently, the time-trends reported by Darden et al. [1] warrant conservative interpretation. Our example may be useful for stimulating further analyses of time-trends in HPV vaccination intent.
REFERENCES
1. Darden PM, Thompson DM, Roberts JR et al. Reasons for Not Vaccinating Adolescents: National Immunization Survey of Teens, 2008-2010. Pediatrics 2013.
2. Centers for Disease Control and Prevention. National, state, and local area vaccination coverage among adolescents aged 13-17 years --- United States, 2009. MMWR Morb Mortal Wkly Rep 2010; 59: 1018-1023.
3. Centers for Disease Control and Prevention. National and state vaccination coverage among adolescents aged 13 through 17 years--United States, 2010. MMWR Morb Mortal Wkly Rep 2011; 60: 1117-1123.
4. Rothman KJ, Greenland S, Lash TL. Modern Epidemiology. Philadelphia, PA: Lippincott Williams & Wilkins 2008.
Conflict of Interest:
None declared