Children who receive two separate vaccinations against chickenpox may have better protection against the disease compared to those who receive only a single shot, according to researchers in the United States. Their study discovered that immunity from the initial dose can weaken over time.
These findings are particularly significant for Canada, where children between the ages of 12 months and 12 years typically receive a single-dose varicella vaccination. Adolescents and adults in Canada receive two doses, spaced at least four weeks apart.
In the United States, children have been receiving a single dose for over a decade, but recently the recommended number of shots was increased. The first dose is now given at 12 to 15 months old, followed by a second dose at four to six years old.
Dr. Jane Seward, a co-author of the study and acting deputy director of the division of viral diseases at the U.S. Centers for Disease Control and Prevention (CDC), stated, “Our data indicate there is some waning of vaccine-induced immunity over time.”
The study analyzed data from over 350,000 individuals in California between 1995 and 2004. Among them, more than 11,000 developed chickenpox, with nearly 1,100 experiencing the illness despite being vaccinated, a phenomenon known as “breakthrough disease.”
By focusing on children aged eight to twelve, the researchers discovered a significant increase in the rate of breakthrough disease as the time since vaccination increased. Additionally, children who had been vaccinated five or more years prior were 2.6 times more likely to have moderate to severe chickenpox compared to those vaccinated within five years.
“We found that the longer the time since vaccination, the more likely individuals were to have a more serious form of the disease,” said Seward.
Chickenpox, caused by the varicella virus, primarily affects children and results in intensely itchy red blisters on the skin. However, the disease can lead to complications such as flesh-eating disease, brain swelling, and organ damage.
A single dose of varicella vaccine provides approximately 85% protection, greatly reducing the number of children being hospitalized or dying from complications. However, the study suggests that a second dose could potentially enhance immunity even further, further diminishing the number of cases and severity of the disease.
Seward stated, “That second dose of varicella vaccine could offer increased protection against waning immunity.”
While these findings are significant, further research is needed to determine the ideal dosage and to confirm these results in other populations, according to Seward. She added, “For Canada, I personally wouldn’t rush into implementing a second dose policy based on these data. I would definitely take it into consideration. The one-dose policy has proven to be effective for the past decade, leading to dramatic results.”
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