Summary: Chickenpox as a disease results in 2 deaths per 100,000 cases, and about 2,000 per 100,000 cases of complications. Chickenpox vaccine results in ~0.15 deaths per 100,000 cases (most of which turn out to be due to natural varicella, so the true death rate is lower), and about 67.5 per 100,000 cases of complications. Thus, for children, vaccination is safer than pox parties. However, dormant chickenpox virus from both vaccination or natural infection can cause shingles later in life if the immunity fades, and chickenpox re-exposure helps boost immunity. Thus, for adults who have had chickenpox or chickenpox vaccination, pox parties may have some benefit (though they would get a similar benefit from re-vaccination).
Chickenpox is a usually mild infection, and children between 1 and 14 who get it, and who don't have troubles with the immune system, have cancer, or are taking steroids, have an excellent prognosis (source: Medscape)
Otherwise healthy children with varicella have excellent prognoses. In otherwise healthy children aged 1-14 years, the mortality rate is estimated at 2 deaths per 100,000 cases. The case-fatality rate in the general population is 6.7 cases per 100,000 population. Children with immunocompromised states, however, are at risk for severe disease and death. The mortality rate in children who are immunocompromised is much higher than that in otherwise healthy children. Among children with leukemia, the mortality rate of varicella is 7%.
One study suggested that nearly 1:50 varicella cases are associated with complications.
Among the most serious complications are varicella pneumonia and
encephalitis; both are associated with a high mortality rate. Before
universal vaccination, most varicella-related deaths in the United
States were from associated encephalitis, pneumonia, secondary
bacterial infection, and Reye syndrome. (See Complications.) In
addition, significant concerns have been raised about the association
of varicella with severe invasive group A streptococcal disease.
The disease can be serious in neonates, depending on the timing of
infection in the mother. Varicella during pregnancy can cause various
adverse outcomes for mother and infant, depending on the stage of
pregnancy. Neonatal varicella mortality rates can reach 30%.
There have been deaths following chickenpox vaccination, though the rate of death is 10-fold smaller than for chickenpox itself (0.15 in 100,000 vaccinations), and whenever an autopsy has been performed, it was found that the cause of death was from wild-type chickenpox virus, rather than from the vaccine, and that the patient had undiagnosed cancer or other complications (source: Wise RP, Salive ME, Braun MM, et al. (2000). "Postlicensure safety surveillance for varicella vaccine". JAMA 284 (10): 1271–9.; make sure to read the reports in Box 1)
[The US Vaccine Adverse Event Reporting System (VAERS)] received 6574 case reports of adverse events in recipients of varicella vaccine, a rate of 67.5 reports per 100,000 doses sold. Approximately 4% of reports described serious adverse events, including 14 deaths. The most frequently reported adverse events were rashes, possible vaccine failures, and injection site reactions. Misinterpretation of varicella serology after vaccination appeared to account for 17% of reports of possible vaccine failures. Among 251 patients with herpes zoster, 14 had the vaccine strain of varicella zoster virus (VZV), while 12 had the wild-type virus. None of 30 anaphylaxis cases was fatal. An immunodeficient patient with pneumonia had the vaccine strain of VZV in a lung biopsy. Pregnant women occasionally received varicella vaccine through confusion with varicella zoster immunoglobulin. Although the role of varicella vaccine remained unproven in most serious adverse event reports, there were a few positive rechallenge reports and consistency of many cases with syndromes recognized as complications of natural varicella.
Both chickenpox and the chickenpox vaccine can lead to shingles, a neurological disease that occurs when dormant virus re-emerges as immunity fades. To reduce the risk of shingles, immune people need to be re-exposed to the virus or be re-vaccinated in their 50s. Re-exposure used to happen automatically before vaccination, however, since vaccination has lead to a ~80% decrease in the number of varicella cases, natural re-exposure has become more difficult (see medscape reference below). Thus, there has been an increase in the incidence of shingles. Pox parties can be one way of getting re-exposed to varicella.
Before varicella vaccine use became widespread, 4 million cases of chickenpox were reported annually. National seroprevalence data for 1988-1994 indicated that 95.5% of adults aged 20-29 years, 98.9% of adults aged 30-39 years, and more than 99.6% of adults older than 40 years were immune to varicella. The disease was responsible for 11,000 hospitalizations each year and approximately 50-100 deaths.
The adoption of universal vaccination against varicella in 1995 reduced the incidence of varicella, as well as the associated
morbidity and mortality rates.[5, 6, 7, 8] By 2000, vaccination
coverage among children 19-35 months in 3 communities in Texas,
California, and Pennsylvania had reached 74-84%, and reported total
varicella cases had declined 71-84%. Most of the decline occurred
among children aged 12 months to 4 years; however, incidence declined
in all age groups, including infants and adults.
Currently, fewer than 10 deaths occur per year, most of them in unimmunized people. Although vaccination coverage has exceeded 80%
over the past few years, outbreaks of breakthrough varicella still
occur in schools and daycare centers.[10, 11, 12]