Question explored with the scientific record
What are odds of 2 separate cases of breakthrough varicella in 24 yo vaccinated female
The short version: two separate breakthrough varicella infections in a vaccinated 24-year-old woman are uncommon but not rare, and the evidence shows the vaccine's protection wanes over time, especially after a single dose.
The evidence on breakthrough varicella comes mostly from children, not adults. In a 7-year study of vaccinated children, annual breakthrough rates ranged from 0.2% to 2.3%, with a cumulative rate under 6.5% [1]. A 1990 study of healthy adults found a much higher attack rate: 31% (8 of 26) after household exposure, compared to 13% in leukemic children [5]. That same study showed two doses cut the adult attack rate from 38% to 23% [5]. A 2015 case report documents a 19-year-old who had received two doses of varicella vaccine at age 15 and still developed PCR-confirmed varicella [6].
The key numbers for a 24-year-old woman are these:
| Group | Breakthrough rate after exposure | Source |
|---|---|---|
| Children (annual, 7-year study) | 0.2%–2.3% per year | [1] |
| Healthy adults (one dose) | 38% (5 of 13) | [5] |
| Healthy adults (two doses) | 23% (3 of 13) | [5] |
| Children (cumulative, 7 years) | <6.5% | [1] |
The adult data comes from a single small study (26 adults total) from 1990, funded before the vaccine was widely licensed [5]. That is thin evidence for a modern estimate. The larger pediatric studies show that breakthrough infections are common enough to be a routine surveillance finding: in one active surveillance site, 65% of all varicella cases from 2005–2010 were breakthrough infections in vaccinated people [4]. The vaccine effectiveness in outbreak settings ranged from 44% to 100%, meaning in some outbreaks nearly half of vaccinated children still got sick [3].
Two separate breakthrough infections in the same person are less likely than one, but the mechanism that allows the first—waning immunity, especially after a single dose—also allows the second. The 1995 review noted that protection declines at 0.5% to 1% per year after 5–8 years [2]. A 24-year-old vaccinated in childhood is well past that window.
My call: two separate breakthrough varicella infections in a vaccinated 24-year-old woman are plausible and not surprising, especially if she received only one dose or was vaccinated more than 10 years ago. The evidence is too thin to give a precise probability, but the adult attack rate of 23–38% after exposure makes a second event entirely expected. Confidence: moderate.
Sources used 6
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Perspectives on the History and Epidemiology of the Varicella Virus Vaccine and Future Challenges
A comprehensive narrative review tracing the history, epidemiology, and future challenges of varicella-zoster vaccination, including vaccine strains, breakthrough infections, shingles incidence, and next-generation vaccines (subunit and mRNA) with implications for public health …
DOI: 10.3390/pathogens14080813 -
Another vaccine, another treadmill?
Universal infant varicella vaccination may shift most varicella cases to adults and pregnant women, potentially increasing serious health costs despite reduced work absenteeism, and should be considered only after measles control and with clearer evidence on vaccine protection d…
DOI: 10.1111/j.1440-1754.1995.tb02900.x -
Impact of Varicella Vaccine on Varicella-Zoster Virus Dynamics
A comprehensive CDC-era review detailing the impact of routine varicella vaccination on varicella-zoster virus dynamics in the United States, including vaccine effectiveness, breakthrough disease, herpes zoster trends, laboratory confirmation methods, and surveillance limitation…
DOI: 10.1128/cmr.00031-09 -
Comparing Active and Passive Varicella Surveillance in Philadelphia, 2005–2010: Recommendations for the Transition to Nationwide Passive Varicella Disease Surveillance
Active varicella surveillance yielded more accurate case classification than passive surveillance, yet incidence trends were similar, informing the transition to nationwide passive varicella surveillance and highlighting recommendations to improve data collection and laboratory …
DOI: 10.1177/003335491412900108 -
Live Attenuated Varicella Vaccine: Protection in Healthy Adults Compared with Leukemic Children
This study compares the protection against varicella infection in leukemic children and healthy adults immunized with live attenuated varicella vaccine, revealing that natural infection provides superior immunity compared to vaccination.
DOI: 10.1093/infdis/161.4.661 -
Reinfection of Varicella zoster in a vaccinated adult
This study presents a case of reinfection with Varicella zoster virus in a vaccinated adult, highlighting the potential for breakthrough infections despite prior vaccination.
DOI: 10.1016/j.mjafi.2014.03.011