Since the chickenpox vaccine became available in the U.S. in 1995, there has been a dramatic reduction in chickenpox cases. Now, a new study published in the Journal of the Pediatric Infectious Diseases Society shows that hospitalizations and outpatient visits for chickenpox have continued their decline after a second dose of the vaccine was recommended to improve protection against the disease. The findings also indicate that increasing vaccination coverage against the once common childhood illness also confers protection to those who are not immunized themselves.
Chickenpox, also known as varicella, is a highly contagious and sometimes serious disease caused by the varicella-zoster virus. In people who are not vaccinated, it typically causes a blister-like rash, itching, fatigue, and fever. However, chickenpox can also result in serious complications, including pneumonia, meningitis, encephalitis, and even death. Before the vaccine was available in the U.S. in 1995, about 4 million people nationwide were infected with chickenpox each year, according to the Centers for Disease Control and Prevention (CDC). Nearly 11,000 people were hospitalized annually, and 100 to 150 people — mostly children — died of the disease. In 2006, health officials recommended adding a second dose of the vaccine to boost its effectiveness.
In this latest study, CDC researchers Jessica Leung, MPH, and Rafael Harpaz, MD, MPH, used national health care claims data to investigate rates of chickenpox infection from 1994 to 2012. The results showed that hospitalizations for chickenpox have dropped by 93 percent since the vaccine was introduced in 1995. Furthermore, after the introduction of the two-dose varicella vaccination in 2006, hospitalizations declined 38 percent, indicating that the second dose significantly increased the vaccine’s effectiveness.
Outpatient visits for the illness also dropped significantly, the study showed. Overall, there were 84 percent fewer outpatient visits in 2012 compared to the pre-vaccination period, with a 60 percent drop since the two-dose vaccination was recommended in 2006.
“We found that rates for varicella in the U.S. continued to decline as the varicella vaccine program has become fully implemented,” the authors wrote. “We saw significant declines in rates of varicella after the one-dose vaccine was recommended in 1995 in the U.S., and we’re continuing to see additional declines in varicella after two doses were recommended in 2006.”
The largest declines were among children and adolescents 1 to 19 years old, a population targeted for vaccination against chickenpox. But the researchers also saw substantial declines in outpatient visits and hospitalizations among infants younger than 12 months, for whom the vaccine is not yet recommended, and in adults, who are often not immunized. This strongly indicates that both groups have gained some protection against the disease through herd immunity — a type of “community immunity” that occurs when a majority of the population is immunized, thereby containing the spread of disease and minimizing the risk of infection for all community members, even those who aren’t vaccinated.