Varicella vaccine

Varicella vaccine

Vaccine description
Target disease	Varicella (chickenpox)
Type	Attenuated virus
Clinical data
MedlinePlus	a607029
Pregnancy cat.	?
Legal status	℞ Prescription only
Identifiers
ATC code	J07BK01
N(what is this?)  vaccine (verify)

The varicella vaccine is a live (attenuated) virus that protects against the viral disease commonly known as chickenpox caused by Varicella Zoster Virus (VZV). Varicella vaccine is marketed as Varivax in the U.S. by Merck. Another vaccine that is known as Zostavax and used to reduce the risk of shingles (also called Herpes zoster) and Postherpetic neuralgia is caused by the same virus, Varicella Zoster Virus (VZV), and is simply a larger-than-normal dose of Varivax.

Dangers of chickenpox

Prior to the introduction of vaccine in 1995 in the US (released in 1988 in Japan & Korea), there were around 4,000,000 cases per year in the U.S., mostly children, with typically 10,500–13,000 hospitalizations (range, 8,000–18,000), and 100–150 deaths each year.^[1][2] 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 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,^[3][4] though the rate of shingles infection has increased for the same reason.^[5][6][7] Ten years after the vaccine was recommended in the U.S., the CDC reported as much as a 90% drop in chicken pox cases, a varicella-related hospitalization decline of 71%^[1] and a 97% drop in chicken pox deaths among those under 20.^[8] In 2006, the shingles vaccine was determined to be effective at preventing shingles (zoster) in persons 60 years of age and older, if administered regularly.^[9] In 2011, the FDA approved Zostavax for those 50 to 59 years old.^[10]

Vaccination worldwide

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.

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, with 48 of those states also offering philosophical and/or religious exemptions. The vaccination is not routine in the United Kingdom. In the U.S. opinions that it should be dropped (individually or along with all immunizations) are also sometimes voiced.

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. 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. It is feared that there would be a greater number of cases of shingles in adults, until the vaccination was given to the entire population—because adults who have had chickenpox as a child are less likely to have shingles in later life if they have been exposed occasionally to the chickenpox virus (for example by their children). This is because the exposure acts as a booster vaccine.^[11][12]

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.^[13]

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.^[14] Cases of vaccine-related chickenpox have been reported in patients with a weakened immune system,^[14][15] but no deaths.

The literature contains several reports of adverse reactions following varicella vaccination,^[16] including vaccine-strain zoster in children and adults.^[17][18] 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.

Administration

This vaccine is a shot given in the fatty tissue. 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.^[19]

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.^[20]

Side effects

Serious side effects are very rare. However, in rare occasions, some severe reactions have been reported such as seizures, pneumonia, and anaphylaxis.^[21]

The possible mild side effects include redness, stiffness, and soreness at the injection site, as well as fever. A few people may develop a mild rash which usually appears around the shot's area.^[22]

There is a risk of developing herpes zoster (shingles) following vaccination. However, this risk is less than the risk due to a natural infection resulting in chickenpox. Most of the cases reported have been mild and have not been associated with serious complications.^[23]

Effectiveness

According to the American Academy of Pediatrics, clinical trials have demonstrated that varicella vaccine is 70% to 90% effective for preventing varicella and more than 95% effective for preventing severe varicella.^[24]

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.^[25]

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.^[26]

Controversy

Mortality due to primary varicella has declined significantly in countries which make wide use of the varicella vaccine.^[3][14] Zoster (shingles) most often occurs in the elderly. Multiple studies have not observed any effect on zoster incidence. This may be because infants and children immunized in varicella vaccination programs have not yet reached the age where zoster typically occurs. It is likewise too early to observe the effect on postherpetic neuralgia (PHN).

It has been claimed that adult shingles may increase after introduction of varicella vaccine.^[27][28] Preliminary evidence that this has occurred is available from data collected from the Antelope Valley Varicella Surveillance Project (VASP) which began surveillance of shingles in 2000. VASP case reports indicate there was a statistically significant increase in adult shingles cases reported to the Antelope Valley Varicella Active Surviellance Project (VASP) from 2000 to 2003. The 56.1% increase from 237 cases in 2000 to 370 cases in 2002 yields a rate ratio of 1.4 (95% C.I. 1.2-1.7). Increases in cases of shingles reported to VASP occurred in every age category (except 70+) from 2000 to 2001. VASP also reported that verified cases of shingles among adults aged 50 years and older increased 27.5% from 2006 to 2007. (Annual Summary, 2001, 2002, 2003, 2006, 2007 Antelope Valley Varicella Active Surveillance Project, Los Angeles County Department of Health Services; Centers for Disease Control and Prevention (CDC) Cooperative Agreement No. U66/CCU911165-10; Mascola L, et al.) ^[29]

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.^[30]

Duration of immunity

Some vaccinated children have been found to lose their protective antibody in as little as five to eight years.^[31] 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.^[2] 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).^[32]

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" (see below). Historically, exposure of adults to contagious children has boosted their immunity, reducing the risk of shingles.^[28]

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.^{[33][34][35][36][37]}

Immunocompromise

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.^[38]

Pox parties

Main article: Pox party

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 due to the risk of serious injury or death (even in children, as described above) from acquiring the disease. 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.^[39] Lady Montagu imported variolation to England.

See also

• MMRV vaccine

References

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