Studies for Shingrix haven't evaluated efficacy rates for preventing episodes of shingles if only one dose is received. Published studies only report efficacy rates when the complete 2-shot series has been administered.
Pharmacist's Letter, a pharmacy-related information site intended for health care professionals, published the following information based on 'personal communication' with a senior medical information scientist at GSK, the manufacturer of the vaccine:
"Clinical trials have NOT been conducted to evaluate the efficacy of Shingrix if just one dose is received. There is insufficient data available from post-hoc analyses to accurately predict the efficacy of Shingrix in patients that have only received one dose."
You certainly will experience an immune response from one dose of Shingrix, which in theory should increase your protection from shingles, but unfortunately, there isn't a definitive answer regarding how effective it is.
It is therefore important to follow the recommended dosing schedule for Shingrix.
Second Dose Of Shingrix
As a reminder, the following is the recommended dosing schedule for Shingrix (2):
- Two doses (0.5 mL each) administered intramuscularly.
- A first dose at Month 0 (i.e. the initial dose) followed by a second dose administered anytime between 2 and 6 months later.
As stated above, the efficacy of a single dose of Shingrix has not been studied well enough to determine its efficacy versus the complete two-shot series.
What has been studied extensively regarding the Shingrix dosing schedule is the waiting period between dose-one and dose-two.
One large study assessed immune responses (specifically 'humoral immunogenicity') to the Shingrix vaccine with the following dosing schedules:
- First and second doses separated by 2 months (0-2)
- First and second doses separated by 6 months (0-6)
- First and second doses separated by 12 months (0-12)
The study concluded that while all three dosing schedules for Shingrix "elicited robust anti-gE immune responses", the 0-12 dosing schedule (i.e. the second dose given 12 months after the first) did not demonstrate 'non-inferiority', a statistical term that describes a comparison between existing therapies.
All this put another way, getting the second dose of Shingrix anywhere from 2 to 6 months after the first dose (as recommended) should elicit similar immune responses in the body and result in similar efficacy. Getting the second dose after 6 months may not be as effective.
Nevertheless, if it has been more than 6 months after your first dose of Shingrix, the CDC (Centers for Disease Control) recommends you simply get it as soon as possible, and there is no need to restart the vaccine series.
In regard to the second part of your question, the first dose of Shingrix should reduce the likelihood of you experiencing an episode of shingles, but it could certainly still occur.
If you, unfortunately, do experience an episode, Shingrix overall has been shown to reduce the severity of symptoms (although data only exists after the 2-shot series is complete).
The CDC doesn't have a specific recommendation currently regarding how to proceed in getting your second dose of Shingrix if you experience an outbreak/episode in between your first and second dose. They only recommend waiting until the acute stage of the illness is over and symptoms have subsided to be vaccinated again.
If you are experiencing an episode of shingles in-between your Shingrix doses, it would be prudent to discuss options with your doctor.
There have been no clinical trials evaluating the effectiveness of Shingrix after only one dose (as compared to two). It is important to receive the two-shot Shingrix series as directed for maximum efficacy in preventing episodes of shingles.
- Administering Shingrix. CDC
- Immunogenicity, reactogenicity and safety of 2 doses of an adjuvanted herpes zoster subunit vaccine administered 2, 6 or 12 months apart in older adults: Results of a phase III, randomized, open-label, multicenter study. PubMed
- Shingrix Prescribing Information. FDA
- Pharmacist's Letter. Pharmacist's Letter (Subscription Required)