A newly released study shows that the number of the chickenpox cases in the U.S. has significantly dropped since the chickenpox vaccine became available in 1995. This decline was seen even among the largely unvaccinated adult population, a trend that may be explained by the theory of herd immunity.

In order to measure exactly how the introduction of the chickenpox vaccine affected the incidence of the disease, the Center for Disease Control and Prevention analyzed national health insurance claims from 1994 to 2012. In its analysis, the CDC researchers found that there were 93 percent fewer hospitalizations for chickenpox in 2012 and 84 percent fewer outpatient visits for the disease since the vaccine’s introduction.

“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," co-author Jessica Leung explained, according to WebMD.

What is most interesting about the results is that data showed there was even a reduction in the incidence of chickenpox among adults, a demographic less likely to have received the fairly new vaccine. This unexpected trend can be explained by something known as herd immunity. According to Vaccines.gov, the idea of herd or community immunity is that when a large portion of a community is immunized against a contagious disease, there is little opportunity for an outbreak. This protects members of the community who are unable to receive the vaccine, such as pregnant women, young infants, or individuals with compromised immune systems.

"The surrounding population that can be vaccinated are not getting sick, and therefore the data suggest that these infants [that cannot be vaccinated] are also being protected," Leung explained.

Chickenpox is a highly contagious infection caused by the varicella-zoster virus. Along with the classic itchy rash, symptoms of chickenpox usually include fever, headache, tiredness, and loss of appetite. Sometimes chickenpox can cause serious problems that require hospitalization, such as pneumonia and meningitis. If a woman gets chickenpox within the first six weeks of her pregnancy, the virus can also cause serious birth defects — she can even lose the child if she develops the disease closer to her due date. Children are also four times as likely to experience a stroke within six months of being infected with the chickenpox virus. Without widespread immunization programs, such as those offered for the chicken pox, these vulnerable individuals would not be as protected from the virus.

Unfortunately, although the vaccine’s introduction has reduced chickenpox cases, a 2013 study has shown it has done little to reduce cases of shingles, a similar condition caused by the same virus. Up to a quarter of individuals who have had chickenpox go on to develop shingles over their lifetime, but scientists still aren't sure why this happens. According to the study, even as chickenpox immunizations reached 90 percent, shingles continued at the same rate.

Source: Leung J, et al. Impact of the Maturing Varicella Vaccination Program on Varicella and Related Outcomes in the United States: 1994–2012. Journal of the Pediatric Infectious Disease Society. 2015