Shingles: Does adult exposure to chickenpox reduce the risk?

A new study questions the notion that re-exposure to chickenpox in adulthood completely prevents the development of herpes zoster or shingles. New findings show that while the risk falls significantly, full immunization is unlikely. As a result, researchers are calling for a reassessment of childhood vaccination policies.

Being re-exposed to chickenpox as an adult may stave off shingles.
Being re-exposed to chickenpox as an adult may stave off shingles.

Herpes zoster is a viral infection that develops after the reactivation of the chickenpox virus after lying dormant in the body.

Herpes zoster infection is also known as shingles, and varicella virus is also known as the chickenpox virus.

There is a theory that re-exposure to the varicella virus in adulthood is stopping shingles from recurring among people who had chickenpox as children. In other words, reexposure is thought to enhance immunity in adults.

This theory was first proposed by Dr. R. Edgar Hope-Simpson in 1965, and other scientists who later adhered to it call it the “exogenous boosting hypothesis.”

Because of widespread support for this theory, several countries don’t offer children routine vaccination with chickenpox. The issue is that reducing the incidence of chickenpox during adulthood will reduce the protective effects of reexposure, raise herpes zoster infection rates and shift the burden to older populations.

Another explanation some countries— like the United Kingdom — refuse routine vaccination is because chickenpox is perceived to be a “mild” childhood illness. Herpes zoster may meanwhile cause serious complications. The World Health Organization (WHO) estimated in 2014 that the infection caused 4.2 million hospitalizations worldwide each year.

Additionally, new evidence suggests that this boost in immunity isn’t as strong as scientists once thought.

Harriet Forbes, of the Faculty of Epidemiology and Population Health at the London School of Hygiene and Tropical Medicine, UK, is the first and corresponding author of the new study that appears in The BMJ.

33 percent lower risk of shingles in the first 2 years

Forbes and the team analyzed data from 9,604 adults who had been diagnosed with herpes zoster between 1997 and 2018 and had been living with a child who developed varicose veins during that time.

The scientists set out to measure the relative incidence of herpes zoster in the 2 decades following household exposure to chickenpox compared to the baseline incidence — that is, during the unexposed time period.

Of those who attended, 6,584 were women. The adults were, on average, re-exposed to varicella at age 38. Overall, during the baseline time, 4,116 adults developed herpes zoster, and 5,055 during the period of high risk.

The researchers accounted for age, season and calendar time, finding that “adults were 33 percent less likely to develop[ herpes] zoster in the 2 years after household exposure to a child with varicella relative to the baseline era.”

In fact, they were 27 per cent less likely to develop herpes zoster in the 10–20 years following re-exposure to the virus. Men were also more likely to benefit from a boost to the immunity than women.

“The relative incidence of zoster was lower in the periods following exposure to a household contact with varicella,” the authors conclude, “with moderate yet sustained 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 add, indicate that vaccination programs like that of the United Kingdom, which guarantees full immunity for 2–20 years, “may need to be revisited.”

The strengths and limitations of study

The researchers explain they controlled for age, which in the past has proved to be a confounding factor.

Another strength is that the team used a “self-controlled case series” method, i.e. an epidemiological study design that allowed the researchers to control confounding factors and to determine the exact time when the participants were diagnosed with varicella.

Forbes and colleagues, however, also recognize some limitations, including the fact that varicella may be under-recorded in the UK, as many people with the condition do not require a visit to the hospital. This could have contributed to the misclassification of exposed time as unexposed, biasing the results.

In fact, the research did not investigate the risk of being exposed to the virus at work, as could be the case with people working in healthcare or childcare facilities.

The team focused exclusively on household exposure, which may have biased the results, as “About 2 million people, or 5 per cent of England’s population, may have been exposed to varicella children during their work.”

Another disadvantage might be that when varicella is extreme people are only likely to visit their doctors. “Thus,” Forbes and colleagues say, “by observing more severe cases of varicella, our research may have overestimated the degree of exogenous stimulation.”

Finally, the authors agree that the study’s observational nature could not capture causality.

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