Efficacy of Shingrix After 1 Dose
A single dose of Shingrix provides 56.9% effectiveness against herpes zoster, which is substantially lower than the 70.1% effectiveness achieved with the complete 2-dose series—making completion of both doses essential for optimal protection. 1
Single-Dose Effectiveness Data
The largest real-world effectiveness study in Medicare beneficiaries aged >65 years demonstrated that one dose of Shingrix provides 56.9% (95% CI, 55.0-58.8) vaccine effectiveness against herpes zoster 1
This single-dose effectiveness is significantly inferior to the 2-dose series effectiveness of 70.1% (95% CI, 68.6-71.5), representing a meaningful reduction in protection 1
These real-world effectiveness estimates are notably lower than the >90% efficacy demonstrated in clinical trials (97.2% in adults ≥50 years, 89.8% in adults ≥70 years), likely due to differences in outcome specificity and real-world conditions versus controlled trial settings 2, 1
Clinical Trial Data on Incomplete Vaccination
The pivotal ZOE-50 and ZOE-70 trials were designed to evaluate the 2-dose regimen (administered at 0 and 2 months), and efficacy data for a single dose was not a primary endpoint in these registration studies 2
The FDA label explicitly states that Shingrix is administered as a two-dose series, with the second dose given 2 to 6 months after the first dose, and does not provide efficacy claims for single-dose protection 3, 2
Importance of Second-Dose Completion
Real-world data from October 2017 to September 2019 showed that among 7,097,441 first doses administered, only 70% of recipients completed the two-dose series within 6 months, and 80% within 12 months—indicating substantial dropout rates 4
The second dose is critical for achieving the high level of protection (>90% in trials, 70% in real-world settings) that makes Shingrix superior to the older live-attenuated vaccine Zostavax 3, 1
Second doses administered beyond the recommended 6-month window (at ≥180 days) maintain full effectiveness without impairment, so delayed completion is preferable to remaining with only one dose 1
Practical Implications
For immunocompetent adults aged ≥50 years, the second dose should be administered 2-6 months after the first dose, with a minimum interval of 4 weeks 3
For immunocompromised adults aged ≥18 years, a shortened schedule with the second dose at 1-2 months is recommended to provide earlier protection in this higher-risk population 3, 5
Patients who have received only one dose should be strongly counseled to complete the series, as single-dose protection of 56.9% leaves them substantially vulnerable compared to the 70.1% protection from two doses 1
Common Pitfall to Avoid
Do not assume that one dose provides adequate protection—the 13.2 percentage point difference in effectiveness between one and two doses (56.9% vs 70.1%) translates to a meaningful increase in herpes zoster risk for incompletely vaccinated individuals 1
The high reactogenicity of Shingrix (injection-site reactions in 9.5%, systemic symptoms in 11.4%) may deter some patients from returning for the second dose, but these transient side effects should not prevent series completion given the substantial efficacy benefit 3, 6