Chickenpox, or varicella, was once a widespread childhood illness characterized by an itchy, blister-like rash. The varicella vaccine significantly altered the landscape of this disease, offering a way to prevent infection. Despite widespread vaccination, some individuals who have received the vaccine still develop chickenpox, a phenomenon known as “breakthrough varicella.”
Understanding Breakthrough Varicella
Breakthrough varicella refers to an infection with the varicella-zoster virus (VZV) that occurs in a person previously vaccinated against chickenpox. It is also known as “vaccine-modified varicella” and manifests as a milder form of the disease compared to chickenpox in unvaccinated individuals.
A diagnosis of breakthrough varicella is applied when the infection occurs more than 42 days after vaccination, to differentiate it from a mild rash appearing shortly after vaccination. Milder symptoms in breakthrough cases are attributed to partial immunity from the vaccine, which lessens disease severity.
Why Breakthrough Cases Occur
The occurrence of breakthrough cases reflects that no vaccine offers 100% protection. The varicella vaccine is highly effective, with one dose being approximately 81% effective against any clinical disease and over 95% effective against moderate to severe varicella. A second dose provides additional protection, increasing efficacy against all forms of varicella to about 92%, with studies showing up to 98% effectiveness against any varicella and 100% against severe cases.
Several factors contribute to breakthrough infections. Immunity can wane over time, particularly if only one dose was received, although two doses are now the standard recommendation for more lasting protection. Individual variations in immune response mean not everyone will develop the same level of protective antibodies after vaccination. Exposure to a high viral load, such as prolonged close contact with someone with severe chickenpox, may also overcome vaccine-induced immunity in some individuals.
Recognizing and Managing Breakthrough Varicella
Breakthrough varicella presents with milder symptoms compared to a full-blown chickenpox infection in unvaccinated individuals. Patients often experience fewer than 50 lesions, often as few as 10 to 20. These lesions may be more maculopapular (flat, red spots) rather than the classic fluid-filled blisters, or they may not blister at all, sometimes resembling insect bites.
Fever is absent or low-grade, and the overall duration of illness is shorter. Despite milder symptoms, individuals with breakthrough varicella are still infectious and should prevent spread. This includes isolation until all lesions have crusted over, which occurs within 1 to 4 days, shorter than typical chickenpox. Medical advice should be sought for confirmation, and symptoms can be managed with supportive care such as fever reducers and anti-itch remedies.
The Continuing Value of Vaccination
Despite the possibility of breakthrough cases, the varicella vaccine remains an effective tool in public health. The vaccine significantly reduces the incidence of severe chickenpox, hospitalizations, and serious complications such as pneumonia or encephalitis. Even when breakthrough cases occur, they are milder, confirming the vaccine’s protective role in reducing disease severity.
Vaccination also contributes to herd immunity, which helps protect vulnerable populations who cannot be vaccinated, such as infants or immunocompromised individuals. Studies have shown that the vaccine has led to a significant decrease in chickenpox incidence, with rates being nine to ten times lower than in the pre-vaccine era. The two-dose schedule further enhances protection and contributes to long-lasting immunity, reducing the overall burden of the disease.