Parents who refused or delayed childhood vaccinations, even without a medical reason for their kids to skip the shot, are helping fuel outbreaks of measles and pertussis in the country, a new review suggests.

Since the disease was declared wiped out in 2000, over half of 1,416 cases of measles reported in the United States were for individuals with no history of measles vaccination. More than 570 people were unvaccinated despite being eligible for the shot.

Of more than 10,000 pertussis patients with identified vaccination status, 24 to 45 percent of those in the five biggest state-wide epidemics since 1977 were unvaccinated or only partially vaccinated, the analysis shows.

Several pertussis outbreaks, however, also took place in highly vaccinated groups, which indicated waning immunity.

“Fundamental to the strength and legitimacy of justifications to override parental decisions to refuse a vaccine for their child is a clear demonstration that the risks and harms to the child of remaining unimmunized are substantial,” says senior study author Dr. Saad B. Omer of Emory University in Atlanta.

Dr. Omer adds that if there are a high number of at-risk or unvaccinated people, the risk of infection goes up even for vaccinated kids. This is because there are few vaccines that show 100 percent effectiveness, he explains in an email to Reuters.

The researchers have analyzed reports of measles outbreaks in the U.S. since the problem was declared eliminated in 2000, along with data on pertussis since the lowest point of its incidence in 1977.

According to the U.S. Centers for Disease Control and Prevention, up to 2 percent of those who receive the five recommended pertussis or whooping cough vaccine doses on time may still acquire the infection. On the other hand, around 3 percent who get both recommended doses of the measles vaccine may still get the virus.

The authors have found, however, that majority of the cases in the outbreaks analyzed were because of intentionally skipping the vaccines. For instance, 405 out of the 574 unvaccinated had non-medical reasons, including religious objections.

Fewer than 1 percent of children need to skip the shot because of medical circumstances, as shown by data from West Virginia and Mississippi, the only states that grant related exemptions. Data reflect higher vaccination rates in Mississippi than in states with looser regulations.

States require childhood vaccinations through public school admission, with some of them tightening laws on vaccination exemptions. Previous research has shown that the more lenient the state is, the more likely the parents are able to apply for exemption.

In a commentary, Dr. Matthew Davis of the University of Michigan says it is high time we evaluate how the healthcare community can achieve “unprecedented levels” of vaccination access, control waning immunity, and minimize preventable diseases for both adults and children.

"Without a centralized infrastructure focused on the goal of maximizing community immunity, high-reliability vaccine coverage remains challenging in the United States," he warns.

The findings were published in the journal JAMA on Tuesday, March 15.

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