The initial VZV infection results in chickenpox, whereas viral reactivation following a period of dormancy manifests as shingles.
Strategies to reduce both chickenpox and shingles incidence should emphasize the importance of shingles vaccination, while slowly incorporating chickenpox vaccination and minimizing varicella zoster virus (VZV) complications, according to a recent study published in BMC Medicine.
VZV is one of the 8 known human herpesviruses. The initial VZV infection results in chickenpox, whereas viral reactivation following a period of dormancy manifests as shingles.
There are currently separate vaccines that protect against both the initial infection and subsequent reactivation. The shingles vaccine is administered either as a single (Zostovax) or double (Shingrix) dose recombinant vaccine that is given later-in-life to reduce reactivation.
However, there is controversary surrounding chickenpox vaccination, because many countries do not include the vaccine in their childhood immunization schedule because of the hypothesized negative impact on immune boosting. This is defined as VZV reactivation being suppressed through exogenous boosting of VZV antibodies from exposure to natural chickenpox infections, according to the study.
“Although the percent of infections resulting in serious illness is low, when endemic VZV infects the majority, vaccination prevents a substantial number of serious cases (e.g., hospitalizations),” the study authors wrote. “For instance, vaccination prevents an estimated 4 million cases each year in the United States, and 1% of those averted cases (approximately 40,000) would have been serious illness.”
The study looked at population-level chickenpox and shingles data from Thailand, which is a country that does not vaccinate against either disease. The notifications from the country were previously fitted with mathematical models to estimate the rates of VZV transmission and reactivation. Multiple chickenpox and shingles vaccination scenarios were then simulated and compared to a model lacking any vaccination to analyze the long-term impacts of VZV vaccination.
The simulations suggested that introducing the chickenpox vaccine at any coverage level would reduce chickenpox incidence. Further, chickenpox vaccine coverage levels above 35% would increase shingles incidence under realistic estimates of shingles coverage with the current length of protective immunity from the vaccine.
Mid-level chickenpox coverage levels were identified as the optimal target to minimize total zoster burden. Only scenarios in which the shingles vaccine provided lifelong immunity or when coverage exceeded current levels lead to significant declines in both chickenpox and shingles.
A key study limitation was that it analyzed chickenpox and shingles dynamics under various vaccination scenarios in Thailand and did not include infection complications, including VZV caused death in the model.
“Any low- to mid-level chickenpox coverage would lead to an increase in the mean age of chickenpox infection, which could lead to more serious chickenpox complications in unvaccinated individuals if herd immunity were not rapidly achieved and sustained,” the study authors wrote. “However, while chickenpox vaccination would increase shingles incidence, previous work has demonstrated that chickenpox vaccination would reduce the mean age of shingles reactivation, potentially curtailing serious side effects of shingles, as younger individuals tend to have milder symptoms.”
The researchers concluded that balancing the consequences of vaccination to overall health impacts, including understanding the impact of an altered mean age of infection for both chickenpox and shingles, would need to be considered before any vaccine was introduced.
“The observed non-linear relationship between chickenpox coverage and the number of cases prevented could be exploited to minimize both chickenpox and shingles incidence,” the study authors wrote. “Low and high chickenpox vaccine coverage performed similarly in preventing chickenpox cases during the first few years of the simulation, and were nearly identical in the long term while lower chickenpox coverage also prevented excess shingles cases.”
Bakker, K.M., Eisenberg, M.C., Woods, R.J. et al. Identifying optimal vaccination scenarios to reduce varicella zoster virus transmission and reactivation. BMC Med 20, 387 (2022). https://doi.org/10.1186/s12916-022-02534-7