Shingles is a painful viral infection caused by the reactivation of the varicella-zoster virus (VZV), the same virus responsible for chickenpox. After recovery, VZV lies dormant in nerve tissue near the spinal cord and brain. The current vaccine recommended for adults aged 50 and older is Shingrix, a non-live, recombinant vaccine. This highly effective immunization is administered in a two-dose series, typically separated by two to six months.
The Purpose of the Second Dose
The two-dose series creates a robust and lasting defense against the virus. The first injection, or priming dose, introduces a VZV protein and an adjuvant to boost the immune response. This initial exposure alerts the immune system, causing it to recognize the viral protein. The immune system begins producing antibodies and specialized white blood cells, such as CD4 T cells.
The second dose acts as a crucial booster, significantly amplifying the initial immune reaction. This effect is necessary to achieve the high levels of antibodies and T-cell response required for durable, long-term protection. The booster generates a massive increase in T cells, which are the immune memory cells that sustain protection over many years. Without the second dose, the immune response remains incomplete and much weaker, leaving the body vulnerable.
Reduced Efficacy and Risk of Shingles
Skipping the second dose results in a substantial loss of the vaccine’s protective power, directly increasing the risk of developing shingles. While the first dose offers some initial defense, a single dose is not sufficient to generate full, optimal immunity. Clinical trials demonstrated that a complete two-dose series provided greater than 90% effectiveness in preventing shingles for healthy adults aged 50 and older.
In contrast, real-world data indicates that the effectiveness of a single dose is notably lower, often around 57% to 64% in the first year. The limited protection provided by a single dose is not sustained over time and wanes much faster than the full series.
For example, one study showed that the effectiveness of a single dose dropped to 52% after the third year, while the two-dose regimen maintained higher protection. The decreased protection also extends to Postherpetic Neuralgia (PHN), the most common and debilitating complication of shingles. Completing the two-dose series is the best defense against PHN, with two doses being about 76% effective against this complication.
Guidance for Delayed or Missed Doses
If the second shot is missed or delayed beyond the recommended six-month window, the primary guidance is to get the dose as soon as possible. The vaccine series does not need to be restarted, regardless of how much time has passed since the first injection. The protection offered by the first dose is not lost, and receiving the second dose completes the series and activates the full, long-term immune response. Studies show that even when administered past the initial six-month interval, the final two-dose effectiveness remains high. The Centers for Disease Control and Prevention (CDC) advises immediate administration if more than six months have elapsed.