Varicella vaccine

The varicella vaccine protects against the disease commonly known as chickenpox.

Dangers of chickenpox
Chickenpox is most often a mild disease, especially for children. Prior to the introduction of vaccine, there were around 4,000,000 cases per year in the U.S., mostly children, with typically 100 or fewer deaths. Though mostly children caught it, the majority of deaths (by as much as 80%) were among adults. Additionally, chickenpox involved the hospitalization of about 10,000 people each year.

During 2003 and the first half of 2004, the CDC reported eight deaths from varicella, six of whom were children or adolescents. These deaths and hospitalizations have substantially declined in the U.S. due to vaccination, though the rate of shingles infection has increased for the same reason. The vaccine has more recently been determined to be effective at preventing shingles (zoster) in persons 60 years of age and older, if administered regularly.

Vaccination worldwide
Vaccination is common in the United States, with 41 of the 50 states require immunization for children attending government-run schools. All 50 states offer a medical exemption, and 48 of those states also offering philosophical and/or religious exemptions. The vaccination is not routine in the United Kingdom. Debate continues in the UK on the time when it will be desirable to adopt routine chickenpox vaccination, and in the U.S. opinions that it should be dropped, individually, or along with all immunizations, are also voiced.

Japan was among the first countries to routinely vaccinate for chickenpox. Routine vaccination against varicella zoster virus is also performed in the United States, and the incidence of chickenpox has been dramatically reduced there (from 4 million cases per year in the pre-vaccine era to approximately 400,000 cases per year as of 2005). In Europe most countries do not currently vaccinate against varicella, though the vaccine is gaining wider acceptance. Australia, Canada, and other countries have now adopted recommendations for routine immunization of children and susceptible adults against chickenpox. Other countries, such as Germany and the United Kingdom, have targeted recommendations for the vaccine, e.g., for susceptible health care workers at risk of varicella exposure. The long-term duration of protection from varicella vaccine is unknown, but there are now persons vaccinated more than thirty years ago with no evidence of waning immunity, while others have become vulnerable in as few as 6 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, and we are only now entering an era in the U.S. where the long-term efficacy of varicella vaccine can be accurately gauged.

The vaccine is exceedingly safe: approximately 5% of children who receive the vaccine develop a fever or rash, but there have been no deaths yet (as of 1 May 2006) attributable to the vaccine despite more than 40 million doses being administered. Cases of vaccine-related chicken pox have been reported in patients with a weakened immune system, but no deaths.

The literature contains several reports of adverse reactions following varicella vaccination,            including vaccine-strain zoster in children and adults. A mean of 2,350 reports per year are attributed to varicella vaccine based on 20,004 cases reported to the Vaccine Adverse Event Reporting System (VAERS) database from May, 1995 through December, 2003. Minor events are known to be under-reported to VAERS.

Controversy
Mortality due to primary varicella has declined significantly in countries which make wide use of the varicella vaccine. Zoster (shingles) occurs decades after varicella and unsurprisingly zoster incidence has not declined in multiple studies. It is too early to observe the effect on postherpetic neuralgia (PHN).

It has been claimed that shingles may increase after introduction of varicella vaccine. There is yet no evidence this has occurred, and it might occur in the absence of immunisation due to a general decrease in childhood infection for other reasons.

Additional controversy has arisen because cell lines derived from aborted fetal tissue were used in its development, and thus violates the ethics and beliefs of people who oppose the use of aborted fetal tissue in medical research.CDC

Duration of immunity
Some vaccinated children have been found to lose their protective antibody in as little as five to eight years; 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." As time goes on, boosters may be determined to be necessary, and introduced. Persons infected after vaccine experience milder cases of chicken pox.

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" (and similarly for some other diseases such as rubella. See below.)  Historically, exposure of adults to contagious children has boosted their immunity, reducing the risk of shingles. Second episodes of chickenpox have been rare, but occur and probably more frequently in the UK latterly and definitely more frequently in the vaccine group. In one study, 30% of children had lost the antibody after five years, and 8% had already caught "wild" chickenpox in that five year period.

The CDC and corresponding national organisations 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..

Immunocompromise
The mortality rate in immunocompromised patients with disseminated herpes zoster is 5-15%, with most deaths from pneumonia. Vaccines, unfortunately are less effective among these high-risk patients, as well as being more dangerous because it is an attenuated live virus (see last footnote), but clearly immunisation before immunocompromise would be desirable.

Pox parties
A "pox party" is a party held by parents for the purpose of infecting their children with childhood diseases. Similar ideas have applied to other diseases, e.g. measles, but are now discouraged by doctors and health services. The reasoning behind such parties is that guests exposed to the varicella virus will contract the disease and develop strong and persistent immunity at an age before disaster is likely, particularly from chickenpox or rubella.

The first reference to such a practice is the letter of Lady Montagu to Sarah Chiswell describing parties held in Istanbul for the purpose of variolation - an effective technique for gaining immunity to smallpox. Lady Montagu imported the practice to England.

Pox parties have been portrayed in TV cartoons, including South Park ("Chickenpox") and The Simpsons ("Milhouse of Sand and Fog").

An 18th century pox party also features in M.T. Anderson's National Book Award-winning young adult novel, "The Astonishing Life of Octavian Nothing. Volume 1: The Pox Party."