A new study questions the notion that reexposure to chickenpox in adulthood completely prevents the development of herpes zoster, or shingles. The new findings show that while the risk drops significantly, full immunization is unlikely. As a result, the researchers call for a reappraisal of childhood vaccination policies.
Herpes zoster is a viral infection that occurs when the chickenpox virus becomes reactivated after lying dormant in the body.
The herpes zoster infection is also known as shingles, and the chickenpox virus is also called the varicella virus.
There is a theory that, among people who had chickenpox as children, reexposure to the varicella virus in adulthood stops shingles from reoccurring. In other words, the belief is that reexposure boosts immunity in adults.
Dr. R. Edgar Hope-Simpson first formulated this theory in 1965, and other scientists, who have later adhered to it, call it the “exogenous boosting hypothesis.”
As a result of widespread support for this theory, several countries do not offer routine chickenpox vaccination for children. The concern is that reducing the prevalence of chickenpox would limit the protective effects of reexposure during adulthood, increasing the rates of herpes zoster infection and shifting the burden on to older populations.
Another reason that some countries — such as the United Kingdom — avoid routine vaccination is that chickenpox is considered a “mild” childhood disease. Meanwhile, herpes zoster can cause severe complications. In 2014, the World Health Organization (WHO) reported that the infection led to 4.2 million hospitalizations around the world annually.
Furthermore, new evidence suggests that this immunity boost is not as strong as scientists once believed.
Harriet Forbes, from the Faculty of Epidemiology and Population Health at the London School of Hygiene and Tropical Medicine, in the United Kingdom, is the first and corresponding author of the new study, which appears in The BMJ.
Forbes and the team analyzed data from 9,604 adults who had received a diagnosis of herpes zoster between 1997 and 2018 and had lived with a child who developed chickenpox during that period.
The scientists set out to measure the relative incidence of herpes zoster in the 2 decades after exposure to chickenpox in the household, compared with the incidence during baseline — that is, during the unexposed time.
Of the participants, 6,584 were women. The adults were reexposed to varicella at the age of 38 years, on average. Overall, 4,116 adults developed herpes zoster during the baseline period and 5,055 during the high-risk period.
The researchers adjusted for age, season, and calendar time, concluding that “In the 2 years after household exposure to a child with varicella, adults were 33% less likely to develop [herpes] zoster,” compared with the baseline period.
Furthermore, in the 10–20 years after reexposure to the virus, they were 27% less likely to develop herpes zoster. Also, men were more likely to benefit from an immunity boost than women.
“The relative incidence of zoster was lower in the periods after exposure to a household contact with varicella,” conclude the authors, “with modest but long-lasting protective effects.”
“This study suggests that 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 findings, they continue, suggest that vaccination policies such as that of the U.K., which assumes complete immunity for 2–20 years, “may need revisiting.”
The researchers explain that they controlled for age, which has proven to be a confounding factor in the past.
A further strength involves the team having used a “self-controlled case series” method, that is, an epidemiological study design that enabled the researchers to control for confounding factors and determine the precise time when the participants were diagnosed with varicella.
However, Forbes and colleagues also acknowledge some limitations, including the fact that varicella may be under-recorded in the U.K., as many people with the condition do not require a hospital visit. This may have resulted in misclassifying exposed time as unexposed, biasing the results.
Furthermore, the research did not look at the possibility of being exposed to the virus at work, as could be the case for people working in healthcare or childcare.
The team focused exclusively on household exposure, which may have biased the results, as “About 2 million people, or 5% of the population of England, during their work could be exposed to children with varicella.”
A further limitation may be that people are only likely to visit their physicians when varicella is severe. “Therefore,” Forbes and colleagues note, “by capturing more severe varicella cases, our study might have overestimated the degree of exogenous boosting.”
Finally, the authors acknowledge that the observational nature of the study could not capture causality.