The current standard for preventing shingles, a condition caused by the varicella-zoster virus (VZV) reactivation, is the recombinant zoster vaccine, Shingrix. This vaccine is recommended for adults aged 50 and older, and for certain immunocompromised adults aged 19 and older, to protect against the painful rash and its long-term complications. The complete Shingrix regimen involves two doses. Missing the second dose is a common concern, leading many to wonder about their level of protection and their next steps.
Why Two Doses Are Necessary
The two-dose schedule for Shingrix is based on the immunological strategy known as prime-boost vaccination. The first dose, or “prime,” introduces the VZV glycoprotein E (gE) antigen and a specialized adjuvant called AS01B. This initial exposure stimulates the immune system to recognize the antigen and begin building a defense. The adjuvant stimulates robust T cell responses, which are important for fighting VZV reactivation in older adults whose immune systems weaken over time.
The second injection, the “boost,” is designed to significantly amplify the immune response generated by the first dose. This booster increases the quantity of VZV-specific antibodies and strengthens cellular immunity, particularly CD4+ T-cells. Without this boost, the initial immune response is typically short-lived and insufficient to provide the high level of durable protection. Completing the two-dose series ensures the immune defense is strong and persistent against the reactivation of the dormant virus.
The Level of Protection After One Dose
Protection after only one dose of Shingrix is significantly lower and less sustained compared to the complete regimen. Clinical trials show a substantial difference in vaccine effectiveness between one dose and two doses. For adults aged 50 and older, a complete two-dose series provides a high degree of protection against shingles, often cited as being over 90% effective.
A single dose offers only partial and temporary defense against the varicella-zoster virus. Real-world data indicates that while one dose may offer initial effectiveness against shingles, this protection wanes considerably over time. The two-dose effectiveness remains high over several years, but the effectiveness of a single dose drops notably after the first year.
Individuals who have only received one shot remain at a higher risk of developing shingles and its associated complications, such as postherpetic neuralgia (PHN). PHN is a long-lasting nerve pain that occurs after the shingles rash has cleared. The single dose may not provide adequate protection against the severity of the illness or the risk of PHN, which is a major goal of the vaccination series. Relying on a single dose leaves the body vulnerable to VZV reactivation and an increased likelihood of a less favorable outcome.
Guidance for Delayed Second Doses
The recommended window for receiving the second dose of Shingrix for immunocompetent adults is two to six months after the first dose. This timeframe is based on clinical data designed to achieve the strongest and most durable immune response. If this recommended interval is missed, patients should not assume they need to restart the entire series.
The second dose of Shingrix should be administered as soon as possible, regardless of the time elapsed since the first shot. There is no need to repeat the first dose, even if the delay extends past the six-month mark. Studies have shown that the effectiveness of the two-dose regimen is not significantly impaired even when the second dose is delayed beyond six months.
Anyone who has missed the six-month window should contact their healthcare provider or pharmacist to schedule the catch-up dose. Completing the series, even with a delay, is the only way to achieve the high, long-term protection against shingles. The delayed second dose still acts as the necessary boost. This ensures the patient’s immune system achieves the intended level of defense.