Chickenpox, caused by the varicella-zoster virus, was once a common childhood illness. Before the introduction of a vaccine, nearly everyone contracted chickenpox during their early years. The disease typically presented with an itchy, blister-like rash that could cover the entire body. The landscape of this infection significantly changed with the widespread use of the varicella vaccine, which has substantially reduced cases and complications.
Understanding Breakthrough Infections
It is possible to contract chickenpox even after receiving the vaccine, a phenomenon known as a “breakthrough infection.” This occurs when a vaccinated individual gets infected with the wild-type varicella-zoster virus more than 42 days after vaccination. While the varicella vaccine is highly effective, it does not offer 100% protection.
Two doses of the chickenpox vaccine are about 90% to 98% effective at preventing all forms of chickenpox. Post-licensure studies show a two-dose effectiveness of 92%, with a range of 88% to 98%. Breakthrough infections are less frequent among those who have received both recommended doses compared to those who have only received one.
Why Breakthrough Infections Occur
Several factors contribute to breakthrough infections, demonstrating that vaccine protection is not absolute. One reason is an incomplete immune response in some individuals. Not everyone develops the same level of protective antibodies after vaccination, meaning their immune system might not be fully equipped to fend off the virus. Studies indicate some healthy children may not develop protective antibody levels even after one dose.
Exposure to a very high viral load can also overwhelm vaccine-induced immunity. If a vaccinated person is exposed to a significant amount of the varicella-zoster virus, such as through prolonged close contact with someone experiencing a severe case of chickenpox, the partial immunity from the vaccine might not be enough to prevent infection. Effectiveness can also vary from person to person due to individual biological differences, including genetic factors or underlying health conditions that influence how well someone responds to the vaccine.
What a Breakthrough Case Looks Like
Breakthrough chickenpox cases are much milder than the disease in unvaccinated individuals. Patients often experience fewer skin lesions, usually less than 50, compared to the hundreds seen in unvaccinated cases. The rash in breakthrough cases may appear more like maculopapular spots (flat, red bumps) rather than the classic fluid-filled blisters.
Individuals with a breakthrough infection often have no fever or only a low-grade fever, and the duration of their illness is usually shorter. Complications are also less common in vaccinated individuals who experience a breakthrough infection. These milder symptoms result from the partial immunity provided by the vaccine, which helps the body better combat the virus.
Continued Importance of Vaccination
Despite the possibility of breakthrough infections, vaccination against chickenpox remains important. The vaccine significantly reduces the illness’s severity, transforming what could be a severe infection into a much milder, more manageable condition. It also significantly lowers the risk of serious complications associated with chickenpox, such as bacterial skin infections, pneumonia, or encephalitis.
Vaccination plays a role in establishing herd immunity within communities. When a large percentage of the population is immune, it helps protect vulnerable individuals who cannot be vaccinated, such as infants or those with weakened immune systems. High vaccination coverage reduces the overall circulation of the virus, making it more difficult for chickenpox to spread. Therefore, getting vaccinated continues to be the most effective strategy for personal protection and for safeguarding public health against chickenpox.