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British study supports exogenous boosting theory.
A new observational study appears to support the theory that adults who are re-exposed to the varicella zoster virus (VZV) after having chickenpox as a child are less likely to develop shingles, also known as herpes zoster (HZ). However, the study’s authors reported that although the exposure may boost immunity to the illness, it does not provide complete protection.
To review, a primary infection with VZV causes chickenpox, usually during childhood. After the initial infection, the virus lies dormant in the body. Years later, it can reactivate as shingles. Protection against shingles is available through vaccination.
The study, published in The British Medical Journal, is based on a review of general practice and hospital records for 9604 adults in the United Kingdom (UK) who were diagnosed with shingles between 1997 and 2018, and who lived with a child with chickenpox during an average 15-year observation period.
After adjusting for various factors (including age, calendar time, and season), strong evidence emerged suggesting that in the 2 years after household exposure to a child with chickenpox, adults were 33% less likely to develop shingles compared with baseline (unexposed) time. The boosting effect was stronger among men than among women after exposure to varicella.
Ten to 20 years after exposure, the protective effect had declined slightly, but the adults were still 27% less likely to develop shingles compared with baseline time.
The findings validate the Hope-Simpson exogenous boosting hypothesis, which has gained widespread support since it was first introduced in the mid-1960s. It postulates that exposure to children with varicella boosts the VZV immunity of adults, thereby preventing HZ.
The World Health Organization recognizes varicella as “an acute and highly contagious disease” that, “in the absence of a vaccination program, affects nearly every person by mid-adulthood.” It recommends routine vaccination only in countries that can maintain a vaccination rate of at least 80%.
As of 2017, 23 countries worldwide recommend all non-medically exempt children receive the vaccine. Nine recommend it only for high risk groups, 3 additional countries recommend use in certain regions, and other countries make no recommendation.
The United States-where 2 doses of the vaccine are recommended starting at 12 to 15 months of age-has benefited from lower disease burden. “It has reduced the rate of getting chickenpox by about 90%.,” said Jeffrey Cohen, MD and Chief of the Laboratory of Infectious Diseases at the National Institute of Allergies and Infectious Diseases. “Even more interestingly, children who got the chickenpox vaccine have had a reduced rate of zoster of about 70%.”
Most countries in Europe, including the UK, do not routinely vaccinate against varicella. The reasons most often given include the cost commitment for they what consider to be a “mild childhood disease” and concerns that vaccination will shift varicella to older age groups or increase herpes zoster incidence.
In light of their findings, the researchers in the UK study are calling for a review of their country’s childhood varicella vaccination policy.
"These findings cannot be used to justify for or against specific vaccination schedules," the authors wrote. "They do, however, suggest that previous mathematical models estimating the effect of exogenous boosting in childhood varicella vaccination policy in the UK that assume complete immunity for between two and 20 years may need revisiting."
1. Forbes H, Douglas I, Finn A, et al. Risk of herpes zoster after exposure to varicella to explore the exogenous boosting hypothesis: self controlled case series study using UK electronic healthcare data. BMJ. 2020. https://doi.org/10.1136/bmj.l6987