Yes, it is possible to experience an episode of shingles (i.e. herpes zoster) in-between dose one and two of Shingrix, which are recommended to be given 2-6 months apart.
Although the Shingrix vaccine is effective, it
doesn't completely eliminate the risk of a shingles infection, even after getting both doses. A study published in the New England Journal of Medicine reported the following efficacy data: Overall vaccine [Shingrix] efficacy against herpes zoster was 97.2% (95% confidence interval [CI], 93.7 to 99.0; P<0.001). Vaccine efficacy was between 96.6% and 97.9% for all age groups.
As Shingrix isn't 100% effective, it stands to reason that there is a
chance you could, unfortunately, experience an episode between dose one and dose two.
Your risk of shingles
after your first dose of Shingrix is most likely lower than it would be without it. However, there isn't any data available to know just how common it is as studies for Shingrix only evaluated how effective it was in preventing confirmed cases of shingles after the second dose.
Clinical studies have
not been conducted evaluating the efficacy of Shingrix when just one dose is received.
The Centers for Disease Control also states that it is unknown how effective one dose of Shingrix is. Additionally, they specify that individuals
may be at higher risk of a shingles infection if the dosing schedule isn't followed: The vaccine series need not be restarted if more than 6 months have elapsed since the first dose; however, the efficacy of alternative dosing regimens has not been evaluated, data regarding the safety of alternative regimens are limited, and individuals might remain at risk for herpes zoster during a longer than recommended interval between doses 1 and 2.
It is important to note that Shingrix is
not a live vaccine and cannot 'give' you shingles. If you do experience an episode between dose one and two, it's likely just bad luck (or due to other precipitating factors).
It is possible to experience an episode of shingles between dose one and two of the Shingrix vaccine. Although your risk is most likely decreased after your first shot, the incidence rate is unknown. Answer Summary
Immune Response After One Dose
Even though we don't have data on exactly what your risk of developing shingles is after
one dose of Shingrix, small studies in specific populations do show an immune response greater than placebo after getting the first shot in the series.
One such study, done in individuals with hematologic malignancies (i.e. cancers that affect the blood and lymphatic system), showed the following results:
Poster Abstract Session: Herpes Zoster Vaccine
The above graph is certainly a bit difficult to completely dissect but illustrates a
clear immune response after one dose of Shingrix.
The 'M1' label indicates 1 to 2 months after of Shingrix. dose one The blue bar is for Shingrix while the orange bar is for placebo.
The chart also shows how much greater of an immune response there is
after the second dose (M2).
Unfortunately, we don't know how much the immune response after dose one translates to effectiveness in preventing shingles.
Studies show a significant immune response after the first dose of Shingrix and an ever greater response after dose two. However, it is unknown how much only one dose reduces the risk of shingles.
Getting The Second Dose
If you have an episode of shingles
in-between dose one and dose two, you are still recommended to get the second dose. However, you should wait until it has cleared. From the CDC: If a patient is experiencing an episode of herpes zoster, vaccination should be delayed until the acute stage of the illness is over and symptoms abate. Studies of safety and immunogenicity of RZV in this population are ongoing.
If you had the first dose of Shingrix in October, you should still get your second scheduled dose (which per your question is in March) as long as your current episode of shingles has cleared.
The second dose of Shingrix is recommended even if you experience an episode of shingles in-between dose one and dose two. The current episode needs to be cleared first however.
^ Efficacy of an Adjuvanted Herpes Zoster Subunit Vaccine in Older Adults. NEJM
^ Recommendations of the Advisory Committee on Immunization Practices for Use of Herpes Zoster Vaccines. CDC
^ 1344. Immunogenicity and Safety of an Adjuvanted Herpes Zoster Subunit Candidate Vaccine in Adults with Hematologic Malignancies: A Phase III, Randomized Clinical Trial. IDSA
^ Shingrix Prescribing Information. GSK