MALMO, SWEDEN – The unique 20-year U.S. experience with pediatric universal varicella vaccination hasn’t resulted in the anticipated increase in herpes zoster predicted by the exogenous boosting hypothesis, Lara J. Wolfson, PhD, reported at the annual meeting of the European Society for Paediatric Infectious Diseases.
In fact, the opposite has occurred. And this finding – based upon hard data – should be of considerable interest to European health officials who have been considering introducing universal varicella vaccination into their national health care systems but have refrained because of theoretical concerns raised by the venerable exogenous boosting hypothesis, noted Dr. Wolfson, director of outcomes research at the Merck Center for Observational and Real-World Evidence, Kenilworth, N.J.
The exogenous boosting hypothesis, which dates back to the mid-1960s, holds that reexposure to wild circulating varicella virus prevents development of herpes zoster later in life. Conversely, by vaccinating children against varicella, opportunities are diminished for reexposure to wild type virus among adults who weren’t vaccinated against varicella, so the hypothesis would predict an increase in the incidence of herpes zoster that should peak 15-35 years after introduction of universal varicella vaccination.
“The same virus that causes varicella in children later reactivates after going dormant in the dorsal root ganglia, and it reactivates as herpes zoster, which is 10 times more severe than chicken pox and leads to 10 times the health care costs. So if in fact implementing a universal varicella vaccine program would lead to an increased incidence of herpes zoster, this would be a bad thing,” the researcher explained.
However, the predictive models based upon the exogenous boosting hypothesis are built upon scanty data. And the models have great difficulty in adjusting for the changes in population dynamics that have occurred in the United States and Western Europe during the past quarter century: namely, declining birth rates coupled with survival to an older age.
Dr. Wolfson presented a retrospective study of deidentified administrative claims data from the MarketScan database covering roughly one-fifth of the U.S. population during 1991-2016. Her analysis broke down the annual incidence of varicella and herpes zoster in three eras: 1991-1995, which was the pre–varicella vaccination period; 1996-2006, when single-dose universal varicella vaccination of children was recommended; and 2007-2016, when two-dose vaccination became standard.
The first key study finding was that herpes zoster rates in the United States already were climbing across all age groups back in 1991-1995; that is, before introduction of universal varicella vaccination. Why? Probably because of those changes in population dynamics, although that’s speculative. The second key finding was that contrary to the exogenous boosting hypothesis prediction that the annual incidence of herpes zoster would accelerate after introduction of universal varicella vaccination, the rate of increase slowed, then plateaued during 2013-2016, most prominently in individuals aged 65 or older.
“In comparing the pre–universal varicella vaccination period to the one- or two-dose period or the total 20 years of vaccination, what we saw consistently across every age group is that herpes zoster is decelerating. There is actually less increase in the rate of herpes zoster than before varicella vaccination,” Dr. Wolfson said.
Uptake of the herpes zoster vaccine, introduced in the United States in 2008, was too low during the study years to account for this trend, she added.
Most dramatically, the incidence of herpes zoster among youths under age 18 years plummeted by 61.4%, from 88 per 100,000 person-years in 1991-1995 to 34 per 100,000 in 2016.
And of course, varicella disease has sharply declined in all age groups following the introduction of universal pediatric varicella vaccination, Dr. Wolfson observed.
Her study was supported by her employer, Merck.