We used data from the 2019–2023 NHIS [11]. The NHIS is an ongoing cross-sectional household interview survey of the civilian noninstitutionalized population residing within the 50 states and the District of Columbia. This analysis included Medicare beneficiaries aged ≥65 years. We restricted the sample to beneficiaries enrolled in both Medicare Parts A and B or in a MA plan. Because herpes zoster vaccine is routinely recommended for people starting at age 50, some adults received a herpes zoster vaccine before they enrolled in Medicare at age 65, and thus their Medicare prescription drug coverage was irrelevant to their vaccination decision. We determined whether the respondent received a herpes zoster vaccine before age 65 years using the survey question asking respondents the year of the most recent ZVL or RZV vaccination. We excluded 2761 respondents who received a herpes zoster vaccine before age 65 (among them, 1441 respondents had missing information on the most recent year of ZVL or RZV vaccination and 1320 respondents who received a herpes zoster vaccine before age 65 years).

The study population consisted of 33,578 Medicare beneficiaries. The sample composition was stable across years; most beneficiaries were female (55.6 %), non-Hispanic White (78.1 %), had less than a college degree (70.5 %), had income above the poverty level (92.5 %), and were residents of an urban area (82.0 %) (Table 1). Most beneficiaries included in the sample had ≥1 chronic condition (85.4 %), received a flu shot during the past 12 months (71.6 %), made ≥1 well visit within the past year (93.1 %), and had a usual place to go for care (97.8 %). Beneficiaries aged 65–74 years accounted for the largest proportion (57.8 %), followed by those aged 75–84 (32.6 %) and ≥ 85 (9.6 %) years. Most Medicare beneficiaries elected to receive prescription drug coverage (93.5 %). Compared to those with prescription drug coverage, those without prescription drug coverage had a higher proportion not having a college degree (70.0 % versus 77.5 %), family income below the poverty level (7.3 % versus 9.5 %), not having any chronic condition (14.1 % versus 21.5 %), no flu shot during the past 12 months (27.3 % versus 44.4 %), no well visit within the past year (6.4 % versus 14.3 %), not having a usual place of care (1.8 % versus 7.9 %), and living in a rural area (17.7 % versus 22.6 %). The proportion of beneficiaries without prescription drug coverage was the highest among those who did not have a usual place for care (23.0 %), followed by those who did not make any well visit during the past year (13.4 %) and those whose race/ethnicity group was in the other category (13.2 %).

During the study period, there was an increasing trend in the probability of receiving a herpes zoster vaccine, from 35.8 % in 2019 to 52.3 % in 2023; overall, 43.9 % (14,734) of beneficiaries ever received any herpes zoster vaccine and 16.2 % (5448) received RZV vaccine (Table 2). Beneficiaries with prescription drug coverage had a statistically significantly higher proportion of receiving a herpes zoster vaccine than those without prescription coverage; the gap was 19.8 percentage points (PPs) for herpes zoster vaccine and 8.2 PPs for RZV. Compared vaccination coverage across years, the gap was the largest in 2023 (23.3 PPs), the year when the IRA took effect. Compared with non-Hispanic White beneficiaries, the unadjusted probability of receiving a herpes zoster vaccine and RZV vaccine was 17.3 (p < 0.01) and 10.4 PPs (p < 0.01) lower among non-Hispanic Black and was 16.6 (p < 0.01) and 9.2 PPs (p < 0.01) lower among Hispanic beneficiaries. The proportion of receiving a herpes zoster or RZV vaccine was significantly lower among beneficiaries with lower incomes (18.5 and 9.9 PPs lower, p < 0.01) or lower levels of education (18.6 and 12.1 PPs lower, p < 0.01) and among beneficiaries who did not receive a flu shot during the past 12 months (35.1 and 15.1 PPs lower, p < 0.01), who did not have a usual place for care (22.1 and 10.0 PPs lower, p < 0.01) and who had not made a well visit during the past year (15.9 and 5.4 PPs lower, p < 0.01).

After adjusting for beneficiaries’ characteristics and year fixed effects, the gap in the probability of receiving a herpes zoster or a RZV vaccine between beneficiaries with and without prescription coverage dropped from 19.8 to 12.9 PPs (p < 0.01) and from 8.2 to 5.6 PPs (p < 0.01), respectively (Table 3). Although the magnitude of the adjusted difference within each subgroup was smaller than that of the unadjusted difference in Table 2, the findings from the comparison within subgroups were similar.

Using the NHIS data, this study showed that beneficiaries with less education and lower income and beneficiaries who did not receive a flu shot during the past 12 month, who did not make an annual well care visit within the past year, and who had no usual place for care were less likely to elect to receive Medicare prescription drug coverage. The probability of receiving a herpes zoster or a RZV vaccine was 12.9 and 5.6 PPs lower among those without prescription drug coverage than those with prescription drug coverage, respectively.

Another notable finding is that there was an 11.5 PP jump in RZV vaccination, from 12.0 % in 2019 to 23.5 % in 2021; in contrast, herpes zoster vaccination only increased 4.6 PPs, from 35.8 % to 40.4 % during the same time period. This finding is likely due to the ACIP’s recent recommendations for the use of RZV and some older adults who received RZV were also people who previously vaccinated with ZVL. Moreover, ZVL has not been available in the US since November 2020.

Studies have evaluated OOP costs of herpes zoster vaccination among Medicare Part D enrollees. Prior to the IRA, all Medicare PDP and over 90 % of MA-PD enrollees were required to pay OOP costs for Part D-covered vaccines [13]; OOP costs ranged from $0 at the 10th percentile of the cost distribution to $193 at the 90th percentile for a dose of herpes zoster vaccine. In 2021, Medicare beneficiaries were liable for 31.0 % ($211 million) of the total costs that Medicare Part D paid for herpes zoster vaccines [2]. In a survey conducted in 2020, providers reported patients most often declined RZV due to insurance not covering the vaccine cost [14]. The IRA removed Part D enrollees’ cost sharing responsibility and therefore could increase their herpes zoster vaccination uptake.

The Medicare program does not universally provide prescription drug coverage for enrollees. Beneficiaries who do not have prescription drug coverage are still required to pay the full cost for Part D-covered vaccines out of pocket after the IRA implementation. This study provided evidence that there were sizable vaccination disparities in Part D-covered herpes zoster vaccine between Medicare beneficiaries with and without prescription drug coverage and the vaccination gap was the largest in 2023, the year when the IRA took effect, compared to previous years. We expected to see increasing disparities in herpes zoster vaccination according to Medicare prescription drug coverage status because the IRA only addressed the cost barrier faced by Medicare beneficiaries who have prescription drug coverage.

Examining the characteristics of beneficiaries without prescription drug coverage, we found that these beneficiaries were more likely to have lower socioeconomic status and use medical care less frequently. These findings were consistent with existing studies that found having higher education and using medical care services more frequently (e.g., receiving influenza vaccine, more outpatient visits, and having a usual place of care) were associated with higher probability of receive herpes zoster vaccine [15–18]. The finding that herpes zoster vaccination uptake was lower among non-Hispanic Black and Hispanic beneficiaries when compared to non-Hispanic White beneficiaries was also corroborated by existing literature [15,17–21]. After adjusting for beneficiaries’ characteristics and their receipt of influenza vaccine which, to some extent, accounts for unobserved factors (e.g., less confidence or interest in vaccination or lack of perceived need for the vaccine) that affected their decision on whether to receive a herpes zoster vaccine, we showed that Medicare prescription drug coverage associated with higher herpes zoster vaccination uptake.

Our findings showed that when compared to non-Hispanic White beneficiaries, non-Hispanic Black and Hispanic beneficiaries had lower herpes zoster vaccination coverage but the proportion of not having prescription drug coverage was comparable. These findings were likely due to the inclusion of dually eligible beneficiaries as racial and ethnic minority had a disproportionately higher proportion of being dually eligible but dually eligible beneficiaries were automatically enrolled in a Medicare drug plan. This finding suggested that Medicare prescription drug coverage was unlikely to be the main reason leading to racial/ethnic disparities in herpes zoster vaccination among older adults.

Though the national data showed stable and increasing uptake of herpes zoster vaccination during the study period, over half of the older adults included in the study remained unvaccinated. Continued work needs to be done to increase herpes zoster vaccination uptake among older adults. Although this study showed that Medicare prescription drug coverage was a key factor associated with older adults’ vaccination decision, OOP costs are not the only barrier to vaccination. Strategies such as vaccination reminders, increasing public confidence in vaccine safety and effectiveness, expanding education and awareness efforts about the risk of developing herpes zoster, and stronger provider recommendations, are also needed to address other vaccination barriers [14,22,23].

This study was subject to limitations. First, the NHIS is survey data, which could be subject to common survey errors, such as selection bias, reporting errors, and measurement errors [24]. Moreover, the NHIS changed its survey approach during our study period. Prior to the Covid-19 pandemic, the survey was conducted by face-to-face interviews. Between April and June 2020, all interviews were changed to telephone only; and since July 2020, interviews were attempted by telephone first with follow-ups to complete interviews by personal visits [11]. Changes in data collection method may affect survey results. However, focusing on the data after 2020, our main results and conclusions did not change. Second, the NHIS sample only includes civilian noninstitutionalized population, which could have biased the coverage estimates upward or downward. Finally, our analyses assumed respondents’ Medicare status as of the date of the interview and the status had not changed since the interview. However, over 95 % of adults aged 65 years and over were covered by Medicare [25], we believe that this assumption was reasonable and would not affect our conclusions in any significant way.

Costs are a critical barrier to vaccination uptake among adults [26–28]. The IRA provides meaningful financial relief to 47 million older adults who have Medicare Part D, which improves their access to herpes zoster vaccination; however, full vaccination cost is still required for about 6 million beneficiaries who do not have Medicare prescription drug coverage. Differences in Medicare coverage status might increase the already existing disparities in herpes zoster vaccination between beneficiaries with and without prescription drug coverage reported in this study, and for other vaccines also covered under Medicare Part D, such as the newly recommended RSV vaccine. This study showed that having prescription coverage was associated with higher herpes zoster vaccination uptake; removing the cost barriers facing by all Medicare-covered adults would reduce vaccination disparities and ensure equal access to adult vaccination among the Medicare-covered population [13,29].