Shingrix, the recombinant zoster vaccine, is the preferred method for preventing shingles (herpes zoster) and its long-lasting complication, postherpetic neuralgia (PHN). The vaccine is recommended for all adults aged 50 years and older, as well as for adults 19 and older who are immunocompromised due to disease or therapy. The standard protocol involves a two-dose series administered by intramuscular injection. Completing the full series is the only way to achieve the high level of sustained protection necessary for effective prevention.
Why Two Doses Are Necessary
The requirement for two doses is rooted in the mechanics of how the vaccine stimulates the body’s defenses. The first injection acts as a priming dose, introducing the immune system to the specific viral component it needs to recognize, the glycoprotein E antigen. This initial exposure stimulates a preliminary immune response, teaching the body to build a defense against the antigen.
The second dose functions as a booster, required to significantly strengthen and mature this initial immune response. This booster shot helps to generate a much higher concentration of protective antibodies and specialized memory T-cells. This amplified and long-lasting immunity provides the high level of protection against the virus. The vaccine also contains an adjuvant, a substance that enhances the immune response, which is important for older adults whose immune systems naturally respond less vigorously to vaccines.
Reduced Protection After Only One Shot
Receiving only one dose means the immune system never receives the boost needed to establish full protection. While the single dose offers some initial defense, this protection is incomplete, lower, and potentially short-lived compared to the complete series. Clinical data shows a noticeable drop in effectiveness when the series is not finished.
Real-world effectiveness studies demonstrate that a single dose provides a defense against shingles that is markedly lower than the two-dose regimen. While the two-dose series provides effectiveness in the range of 70% to over 90%, a single dose offers effectiveness closer to 57% to 64% in observational studies. This diminished shield translates directly to a greater risk of developing herpes zoster.
The protection offered by one dose wanes more rapidly over time than the full series. One study showed that the effectiveness of a single dose dropped significantly after the third year, while the two-dose regimen maintained strong effectiveness. This quicker decline means that an individual who stops after the first shot is left vulnerable to the painful rash of shingles and the subsequent risk of postherpetic neuralgia (PHN). PHN is a form of chronic nerve pain that can persist for months or years after the rash has healed.
Guidance on Completing the Series
If you have only received the first dose, the most important action is to schedule the second shot as soon as possible. The initial recommendation is to receive the second dose between two and six months after the first. If that window has passed, you do not need to worry that the first dose was wasted or that you must start the series over.
The series does not need to be restarted, regardless of how much time has elapsed since the first injection. There is no established maximum time limit between the two doses. Evidence suggests that a delayed second dose still provides effective protection and does not impair the vaccine’s overall effectiveness.
It is important to contact your healthcare provider or pharmacy to arrange the catch-up shot. Completing the series, even if delayed, is better than remaining only partially vaccinated and under-protected. While the delay may mean a longer period of suboptimal protection, administering the second dose at any point will complete the series and provide the long-term, high-level immunity.