Exposure to children with chickenpox reduces the incidence of shingles in adults 33% over 2 years, and 27% out to 20 years, according to British investigators.
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Being exposed to children with illness due to varicella infection acts as an “exogenous booster” in adults who had chickenpox themselves as children, making shingles less likely, they explained in a BMJ article.
Although that’s good news, it’s been reported previously that exposure to children with chickenpox confers complete protection against shingles in adults for years afterward.
The finding matters in the United Kingdom because varicella vaccine is not part of the pediatric immunization schedule. The United States is the only country that mandates two shots as a requirement for children to attend school.
The United Kingdom, however, is reconsidering its policy. In the past, the exogenous booster idea has been one of the arguments used against mandating the vaccine for children; the concern is that preventing chickenpox in children – and subsequent reexposure to herpes zoster in adults – would kick off a costly wave of shingles in adults.
The study results “are themselves unable to justify for or against specific vaccination schedules, but they do suggest that revised mathematical models are required to estimate the impact of varicella vaccination, with the updated assumption that exogenous boosting is incomplete and only reduces the risk of zoster by about 30%,” noted the investigators, led by Harriet Forbes of the London School of Hygiene and Tropical Medicine.
The researchers identified 9,604 adults with a shingles diagnosis during 1997-2018 who at some point lived with a child who had chickenpox. Data came from the U.K. Clinical Practice Research Datalink, a general practice database.
They then looked at the incidence of shingles within 20 years of exposure to the sick child and compared it with the incidence before exposure and after 20 years, by which time the exogenous booster is thought to wear off. It was a self-controlled case series analysis, “a relatively novel epidemiological study design where individuals act as their own controls. Comparisons are made within individuals rather than between individuals as in a cohort or case control study,” Ms. Forbes and colleagues explained.
After adjustment for age, calendar time, and season, they found that in the 2 years after household exposure to a child with varicella, adults were 33% less likely to develop zoster (incidence ratio 0.67, 95% confidence interval 0.62-0.73), and 27% less likely from 10 to 20 years (IR 0.73, CI 0.62-0.87). The boosting effect appeared to be stronger in men.
“Exogenous boosting provides some protection from the risk of herpes zoster, but not complete immunity, as assumed by previous cost effectiveness estimates of varicella immunization,” the researchers said.
More than two-thirds of the adults with shingles were women, which fits with previous reports. Median age of exposure to a child with varicella was 38 years.
Ms. Forbes and colleagues noted that “the study design required patients with zoster to be living with a child with varicella, therefore the study cohort is younger than a general population with zoster. ... However, when we restricted our analysis to adults aged 50 and older at exposure to varicella, a similar pattern of association was observed, with no evidence of effect modification by age. This suggests that although the median age of our study cohort ... was low, the findings can be generalized to older people.”
There was no external funding for the work, and the lead investigator had no relevant financial disclosures. One investigator reported research grants from GSK and Merck, both makers of chickenpox and shingles vaccines.
SOURCE: Forbes H et al. BMJ. 2020 Jan 22;368:l6987.