Pharmacists play a critical role in getting older Americans to complete their vaccination series against herpes zoster (HZ), but more needs to be done, according to a new study.

The report in the Journal of the American Pharmacists Association points out that the recombinant zoster vaccines (RZV) can protect the population against consequences of varicella zoster virus reactivation, but that optimal immunogenicity with RZV requires completion of the two-dose series within 2 to 6 months from the first dose.

Industry researchers sought to estimate RZV completion rates and adherence with the recommended administration schedule in the general U.S. population age 50 years or older, while also assessing factors influencing completion rates.

To do that, they analyzed open-source pharmacy and medical claims databases over time for adults aged 50 years or older with a first RZV prescription filled between October 2017 and September 2019. The data were linked to other records to determine information such as race.

The study found that cumulative completion rates were 70.41% and 81.80% at 6 and 12 months, respectively. Researchers calculated median time to second dose at about 4 months (4.08-5.13 months) and adherence at 67.62%.

"Completion rates were lower in the medical claims database compared with the pharmacy claims database (48.98% vs. 73.23% at 6 months)," the authors wrote. "Regression models confirmed that pharmacy claim was an independent factor for higher completion rates, while African American race and Medicaid status were associated with lower completion rates. Most comorbidities, including chronic obstructive pulmonary disease and type 2 diabetes mellitus, were associated with lower completion rates."

The study concluded, "Pharmacists contribute substantially to the overall high RZV completion rates in the United States. However, completion rates can be improved, especially in people receiving their first RZV dose at a physician's office. Future strategies should aim at lowering barriers to completing vaccination series in African Americans, Medicaid beneficiaries, and people with comorbidities."

The issue is important because HZ affects approximately 1 million patients a year in the U.S., and the life-long risk of suffering from an episode of HZ is approximately 10% to 30%, according to the article.

The risk is especially high in older people because of weaker immune systems due to age and in those with immunocompromising conditions, the study added.

RZV has received a preferential recommendation from the Advisory Committee on Immunization Practices over the existing live-attenuated zoster vaccine, which was discontinued from the U.S. market. RZV vaccine efficacy of 90% or higher was demonstrated in clinical studies with a 2-month interval between first and second dose administration.

Background information in the report recounts how subsequent immunogenicity studies comparing different dosing schedules found that 2- and 6-month dosing intervals produced equivalent immune responses, but the 12-month interval did not meet the criterion for equivalence with the 2-month dosing schedule.

On the other hand, the authors note, the anti-IgE humoral immune response rates for all examined dosing schedules were high (96.6%, 96.5%, and 94.5% for the 2-, 6-, and 12-month dosing schedules, respectively).

"In a retrospective claims study among Medicare beneficiaries aged at least 65 years, no difference in vaccine effectiveness was observed between people filling their second RZV prescription within up to 180 days or more than 180 days from the first RZV prescription," the researchers wrote. "Nevertheless, to maximize HZ protection in adults aged at least 50 years, high vaccine uptake (first dose), completion rates (second dose), and adherence with the 2-6 months dosing schedule need to be achieved."

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