Varicella vaccine

The long-term duration of protection from varicella vaccine is unknown, but there are now persons vaccinated twenty years ago with no evidence of waning immunity, while others have become vulnerable in as few as six years. Assessments of duration of immunity are complicated in an environment where natural disease is still common, which typically leads to an overestimation of effectiveness.[13]

Some vaccinated children have been found to lose their protective antibody in as little as five to eight years.[14] However, according to the World Health Organization: "After observation of study populations for periods of up to 20 years in Japan and 10 years in the United States, more than 90% of immunocompetent persons who were vaccinated as children were still protected from varicella." However, since only one out of five Japanese children were vaccinated, the annual exposure of these vaccinees to children with natural chickenpox boosted the vaccinees' immune system. In the United States, where universal varicella vaccination has been practiced, the majority of children no longer receive exogenous (outside) boosting, thus, their cell-mediated immunity to VZV (varicella zoster virus) wanes—necessitating booster chickenpox vaccinations.[15] As time goes on, boosters may be necessary. Persons exposed to the virus after vaccine tend to experience milder cases of chickenpox.[16]

Catching "wild" chickenpox as a child has been thought to commonly result in lifelong immunity. Indeed, parents have deliberately ensured this in the past with "pox parties". Historically, exposure of adults to contagious children has boosted their immunity, reducing the risk of shingles.[17] The CDC and corresponding national organizations are carefully observing the failure rate which may be high compared with other modern vaccines—large outbreaks of chickenpox having occurred at schools which required their children to be vaccinated.[18]

Prior to the introduction of the vaccine in 1995 in the United States (released in 1988 in Japan and Korea), there were around 4,000,000 cases per year in the United States, mostly children, with typically 10,500–13,000 hospital admissions (range, 8,000–18,000), and 100–150 deaths each year.[15][3][19] Though mostly children caught it, the majority of deaths (by as much as 80%) were among adults.

During 2003, and the first half of 2004, the U.S. Centers for Disease Control and Prevention (CDC) reported eight deaths from varicella, six of whom were children or adolescents. These deaths and hospital admissions have substantially declined in the US due to vaccination,[20][21] though the rate of shingles infection has increased as adults are less exposed to infected children (which would otherwise help protect against shingles).[22][23][24] Ten years after the vaccine was recommended in the US, the CDC reported as much as a 90% drop in chickenpox cases, a varicella-related hospital admission decline of 71%[19] and a 97% drop in chickenpox deaths among those under 20.[25]

Vaccines are less effective among high-risk patients, as well as being more dangerous because they contain attenuated live virus. In a study performed on children with an impaired immune system, 30% had lost the antibody after five years, and 8% had already caught wild chickenpox in that five-year period.[26]

Herpes zoster (shingles) most often occurs in the elderly and is only rarely seen in children. The incidence of herpes zoster in vaccinated adults is 0.9/1000 person-years, and is 0.33/1000 person-years in vaccinated children; this is lower than the overall incidence of 3.2–4.2/1000 person-years.[27][28]

Adult shingles cases may increase after introduction of varicella vaccine, but evidence is unclear.[17][23][28][29] While research using computer models has tended to support the hypothesis that vaccination programs would increase incidence of zoster in the short term, the evidence from epidemiological studies is mixed,[30][31] and increases observed in zoster incidence in some studies may not be related to vaccination programs, as the incidence increases prior to the varicella vaccine program being initiated.[32]

Regarding herpes zoster, the U.S. Centers for Disease Control and Prevention (CDC) stated in 2014: "Chickenpox vaccines contain weakened live VZV, which may cause latent (dormant) infection. The vaccine-strain VZV can reactivate later in life and cause shingles. However, the risk of getting shingles from vaccine-strain VZV after chickenpox vaccination is much lower than getting shingles after natural infection with wild-type VZV."[2]

The World Health Organization (WHO) recommends one or two doses with the initial dose given at 12 to 18 months of age.[3] The second dose, if given, should occur at least one to three months later.[3] The second dose, if given, provides the additional benefit of improved protection against all varicella.[33] This vaccine is a shot given subcutaneously (under the skin). It is recommended for all children under 13 and for everyone 13 or older who has never had chickenpox.

In the United States, two doses are recommended by the Centers for Disease Control and Prevention (CDC). For a routine vaccination, the first dose is administered at 12 to 15 months of age and a second dose at age 4–6 years. However, the second dose can be given as early as 3 months after the first dose. If an individual misses the timing for the routine vaccination, the individual is eligible to receive a catch-up vaccination. For a catch-up vaccination, individuals between 7-12 years old should receive a 2 dose series 3 months apart (a minimum interval of 4 weeks). For individuals 13-18 years old, the catch-up vaccination should be given 4 to 8 weeks apart (a minimum interval of 4 weeks).[34] The varicella vaccine did not become widely available in the United States until 1995.[35]

In the UK, the vaccine is only recommended in people who are particularly vulnerable to chickenpox.[6] The National Health Service cites concerns about unvaccinated children catching chickenpox as adults when it is more dangerous, an increased risk of shingles in adults due to the lack of contact with chickenpox-infected children providing a natural boosting to immunity, and the fact that chickenpox is usually a mild illness.[6]