|Target disease||Varicella (chickenpox), herpes zoster, postherpetic neuralgia, Ramsay Hunt syndrome type II|
Varicella vaccine is 70% to 90% effective for preventing varicella and more than 95% effective for preventing severe varicella. Furthermore, follow-up evaluations took place in the United States of children immunized that revealed protection for at least 11 years. Also, studies were conducted in Japan which indicated protection for at least 20 years.
People who do not develop enough protection when they get the vaccine may develop a mild case of the disease when in close contact with a person with chickenpox. In these cases, people show very little sign of illness. This has been the case of children who get the vaccine in their early childhood and later have contact with children with chickenpox. Some of these children may develop a mild chickenpox also known as breakthrough disease.
Duration of immunity
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 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.
Some vaccinated children have been found to lose their protective antibody in as little as five to eight years. However, according to the  Some persons exposed to the virus after vaccine can experience milder cases of chicken pox (and usually then harbor both the attenuated vaccine or oka strain as well as the wild type or natural chickenpox strain which are both subject to reactivation as shingles).
The mortality rate in immunocompromised patients with disseminated herpes zoster is 5–15%, with most deaths from pneumonia. Vaccines are less effective among these high-risk patients, as well as being more dangerous because they contain attenuated live virus (see last footnote). 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.
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.
Adult shingles cases may increase after introduction of varicella vaccine, but evidence is unclear. 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, 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.
Regarding herpes zoster, the US Centers for Disease Control states:
"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." 
Rates of chickenpox
Prior to the introduction of the vaccine in 1995 in the USA (released in 1988 in Japan & Korea), there were around 4,000,000 cases per year in the US, mostly children, with typically 10,500–13,000 hospital admissions (range, 8,000–18,000), and 100–150 deaths each year. 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 US 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, though the rate of shingles infection has increased as adults are less exposed to infected children (which would otherwise help protect against shingles). Ten years after the vaccine was recommended in the US, the CDC reported as much as a 90% drop in chicken pox cases, a varicella-related hospital admission decline of 71% and a 97% drop in chicken pox deaths among those under 20.
The Varicella Zoster vaccine is made from the Oka/Merck strain of live attenuated varicella virus. The virus was initially obtained from a child with natural varicella, introduced into human embryonic lung cell cultures, adapted to and propagated in embryonic guinea pig cell cultures, and finally propagated in human diploid cell cultures.
Japan was among the first countries to 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 the United Kingdom, have targeted recommendations for the vaccine, e.g., for susceptible health care workers at risk of varicella exposure. In the UK, varicella antibodies are measured as part of the routine of prenatal care, and by 2005 all National Health Service personnel had determined their immunity and been immunized if they were non-immune and have direct patient contact. Population-based immunization against varicella is not otherwise practiced in the UK.
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.
Two doses are always recommended. In the first case a first dose is administered at 12 to 15 months and a second dose at age 4–6 years. For people older than 13 the two doses are administered 4 to 8 weeks apart.
The varicella vaccine is not recommended for seriously ill people, pregnant women, people who have experienced a serious allergic reaction to the varicella vaccine in the past, people who are allergic to gelatin, people allergic to neomycin, people suffering from an immune system disease, such as HIV, people receiving high doses of steroids, people receiving treatment for cancer with x-rays, drugs or chemotherapy, as well as people who have received blood products or transfusions during the past 5 months.
Serious side effects are very rare. From 1998 to 2013, only one vaccine-related death was reported: an English child with pre-existent leukemia. In some occasions, severe reactions have been reported such as meningitis and pneumonia (mainly in inadvertently vaccinated immunocompromised children) as well as anaphylaxis.
There is a short term risk of developing herpes zoster (shingles) following vaccination. However, this risk is less than the risk due to a natural infection resulting in chickenpox.:378 Most of the cases reported have been mild and have not been associated with serious complications.
Generally the vaccine is exceedingly safe: approximately 5% of children who receive the vaccine develop a fever or rash, but as of 1 May 2006, there have been no deaths yet attributable to the vaccine despite more than 40 million doses being administered. Cases of vaccine-related chickenpox 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.
A controversy has arisen, in part, because of misunderstandings concerning the role of human embryonic cell lines and vaccine development. Unlike bacteria or fungi, viruses cannot reproduce autonomously and require a living organism host in order to replicate. The ability of the virus to cause infection usually depends on the host with whom its cells have adapted to over generations of replication. In other words, a virus can lead to a more severe infection in certain organisms versus others. To create a vaccine that is safe for individuals while having the ability to induce a protective immune response necessitates choosing an appropriate cell line to grow a virus. WI-38 and MRC-5 are embryonic lung cell lines, derived from two post-mortem fetuses nearly 35 years ago, that have been maintained in laboratory conditions to self-replicate and produce more embryonic lung cells that are used for research purposes. These cell lines cannot develop into a human organism and have no potential for human life. Furthermore, no additional tissue is required from post-mortem fetuses. One of the uses of these cell lines is to provide an environment for virus replication. The Oka/Merck strain of live attenuated varicella virus is one of the viruses that require this environment to grow. Some controversy has arisen due to the mistaken idea that the vaccine contains components of aborted fetus, thus violating the ethics and beliefs of those who oppose the use of aborted fetuses in medical research. In reality, cell line only provides the environment to grow the vaccine and is isolated from cell itself, free from extraneous biological agents. However, other realms of controversy revolve around the use of the WI-38 and MRC-5 cell lines in growing the varicella virus since these human embryonic lung cells were derived from post-mortem fetuses that were aborted. Due to beliefs about the use of human embryonic fetal tissue in medical research, these individuals are opposed to using the varicella vaccine.
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