A handful of major measles outbreaks have underscored the importance of childhood vaccination, but experts say other diseases, chiefly pertussis, could also come back in force.
In theory, the process is simple. Public health agencies set recommended vaccine schedules, children receive vaccinations during well visits and, as a result, some of the most pernicious diseases known to man fade into history.
Yet, for the past 2 decades—ever since a dubious paper questioning the safety of vaccines was published in a premier medical journal—that theoretical equation has been complicated.
Vaccinations have become optional in the minds of some parents, and scary in the minds of others. The result has been a series of outbreaks of preventable diseases, most recently measles.
Public health experts say measles won’t be the last of the outbreaks if vaccination rates continue to lag.
“Measles is the canary in the coalmine,” Daniel Salmon, PhD, MPH, a professor at Johns Hopkins University’s Bloomberg School of Public Health, told Contagion®.
Even though measles disease was thought to be eliminated in the United States, it keeps popping up thanks to low vaccination rates. Dr. Salmon said often the introduction happens because someone from a locality with high levels of vaccine skepticism travels to another part of the world, gets infected, and then returns home where the infection spreads in his or her community.
Christopher J. Gill, MD, an associate professor of public health at Boston University, said other preventable illnesses besides measles have been undergoing resurgences.
“We are already seeing outbreaks of pertussis (whooping cough) linked to undervaccination,” Dr. Gill told Contagion®. “It is the most contagious bacterial disease known. Measles is the most contagious viral disease known.”
According to the US Centers for Disease Control and Prevention (CDC), there were 48,277 cases of pertussis, better known as whooping cough, in the United States in 2012. That’s the highest number seen in the nation since 1955.
More recently, there were more than 13,000 cases in the US last year, and nearly 19,000 cases the year before that, according to CDC surveillance data.
At its lowest point, in 1976, there were just over 1000 cases of pertussis in the country. As with measles, the impact of vaccination is well documented in the scientific literature. A global study from 1998 found that the rates of pertussis in countries with high vaccination rates were 10 to 100 times lower than in countries with low vaccination rates. It found the phenomenon held true even when a country with a high vaccination rate was right next door to a country with a poor vaccination rate.
Despite the evidence for vaccines, and the painful consequences when preventable outbreaks occur, public attitudes can be hard to change. A 2014 study examining a pertussis outbreak in Washington state in 2011-2012 found no significant increase in pertussis vaccination rates in that state after the outbreak. Back then, as many as 1 in 10 kindergarteners in Washington were estimated to have non-medical vaccine exemptions, and during the 2011-2012 outbreak the incidence of pertussis in Washington state was nearly 5 times that of the national average.
Though public health officials uniformly agree that vaccination is the best way of preventing pertussis, there’s no way a person can completely eliminate his or her risk of getting the disease.
“Pertussis is more complicated because waning immunity of the vaccine (protection afforded by the vaccine decreases as someone ages) is also an issue,” Dr. Salmon said.
A 2018 study attempted to delve more deeply into which particular factors were at play in the resurgence of pertussis. Matthieu Domenech de Cellès, PhD, of France’s Pasteur Institute, wanted to see if the latest generation of pertussis vaccines might be to blame for the resurgence of the disease.
To study the question, Dr. Domenech de Cellès and colleagues used 16 years’ worth of pertussis data from Massachusetts and evaluated it against transmission models based on vaccine failure hypotheses. They concluded that the resurgence was the result of incomplete vaccine coverage and the waning over time of vaccine effectiveness. The vaccine itself, though, worked.
“We found evidence that the vaccine itself is effective at reducing overall transmission, yet that routine vaccination alone would be insufficient for elimination of the disease,” Dr. Domenech de Cellès and colleagues wrote in Science Translational Medicine.
Dr. Domenech de Cellès said schoolchildren are the most common transmitters of the disease, and thus should be the target of public health efforts aimed at curtailing pertussis.
Indeed, although multiple factors can be at play in the transmission of infectious diseases, a case from the 1970s shows the dramatic power of vaccines. The CDC notes that when pertussis vaccination rates dropped to 10% in Japan in the latter half of the 1970s, the country went from having 393 cases of pertussis in 1974 to more than 13,000 in 1979. During the same time frame, they went from zero deaths from pertussis to 41 deaths.
Other diseases are also becoming threats again for the first time in a while. Small outbreaks of the mumps have been reported in the United States in recent years, most recently at Temple University in Philadelphia. And in 2006, more than 6500 cases of mumps were reported across the country. That outbreak was linked not only to vaccine refusals, but also to incomplete vaccination (taking only 1 dose), and to the vaccine’s less-than-perfect performance.
Chickenpox is another disease with the potential to make a comeback. Although rates of chickenpox have declined dramatically since a vaccine became available in 1995, many people still refuse the vaccine. In some cases, parents refuse the vaccine because they don’t consider the disease sufficiently dangerous. Some parents even intentionally expose their children to the potentially fatal disease at so-called “Chickenpox Parties.”
Though the public health consequences of waning vaccination rates are well understood, the fight against anti-vaccination campaigners continues.
Dr. Salmon said some vaccine skeptics might be swayed by outbreaks of preventable illnesses, but many will not.
“About 1% of people are really anti-vaccine—ideologically opposed to vaccines,” he said. “It is unlikely that measles outbreaks will change their mind.”
Dr. Salmon estimates that about another third of parents have concerns about vaccines but aren’t necessarily ideologically opposed to them. He said it’s possible the measles outbreaks will cause those parents to vaccinate their children.
Dr. Gill noted that the vaccine movement is generally tied to a single, disproven and retracted study that falsely linked vaccines to autism. Subsequent large-scale studies have repeatedly shown no link between the measles mumps rubella (MMR) vaccine and autism. The most recent, published just this month, involved more than 650,000 children. It found no link between the vaccine and autism.
“If anything, the study found that the autism risk was slightly reduced among the MMR recipients,” Dr. Gill noted. “Interestingly, the study found that their risk stratification scheme was very good at identifying kids who would go on to develop autism (which adds face validity to the study).”
One risk factor was maternal smoking during pregnancy, a behavioral factor that could be reduced with better public awareness. Dr. Gill said he’s seen no effort by anti-vaccination campaigners to discourage maternal smoking during pregnancy.
“Why not? If their goal was actually to reduce autism, you’d think they’d be delighted by this news,” he said. “But, in fact, that is not the case. Their agenda is to stop vaccines and the debate about autism and MMR is a tool to achieve that end.”