A Monday afternoon session at the National Foundation for Infectious Diseases Annual Conference on Vaccine Research was entitled “Current Challenges in Immunization Policy.” The topics ranged from vaccine hesitancy, effectiveness of acellular pertussis vaccine, and the burden of adverse events from rotavirus vaccination.
Saad B. Omer, MBBS, MPH, PhD, of the Emory Vaccine Center, is widely published on vaccine refusal and its consequences in relation to disease. Most of the work he presented Monday explored the relationship between vaccine hesitancy and pertussis incidence. Pertussis in particular is a concern because in recent years we’ve seen greatly increased numbers of pertussis cases in certain states, especially in the western part of the United States. Omer summarized studies from California that show overlap between clusters of exemptions from required school vaccinations and clusters of pertussis incidence. A Glanz study (2013) of data from the Vaccine Safety Datalink project demonstrated that as the magnitude of undervaccination increased, the risk of pertussis also increased. In fact, the risk of contracting pertussis was 28 times higher for children with no documented pertussis vaccination. Another study of data from Colorado showed a six times higher risk of pertussis for exempt children versus nonexempt children.
Omer noted that these observations aren’t particularly novel: studies over 15 years have shown that vaccine refusal is associated with pertussis. A more specific question to study is, “What is the relative contribution of vaccine refusal to the burden of pertussis ”? Though he couldn’t quantify this, Omer said that he’d suppose that while vaccine refusal is a contributor, it’s almost definitely not the top reason for resurgence of pertussis. In other words, if we were able to reduce pertussis vaccine refusal, we will still not eliminate the pertussis burden we’re seeing now: other problems, such as the suboptimal protection offered by the acellular pertussis vaccine, contribute to the situation. Still, if there are opportunities to reduce vaccine refusal, we should pursue them, with the knowledge that doing so won’t put an end to the pertussis problem.
Looking more specifically at the epidemiology of pertussis, Thomas A. Clark, MD, MPH, of the CDC, addressed the topic “Acellular Pertussis Vaccines: Are Boosters Indicated?” With the introduction of the less reactogenic, and less effective, acellular pertussis vaccine in the 1990s, we’ve seen a resurgence of pertussis cases in the United States. Our historic lows occurred in the 1970s, when we experienced about 1,000 pertussis cases a year. But now, about 20 years after the introduction of the acellular vaccine, we’ve seen a high of about 48,000 cases a year in 2012 and 24,000 cases in 2013 (see this graph of pertussis cases by year for the full picture). This is in spite of the fact that we’ve introduced a booster dose of pertussis-containing vaccine for adolescents and we’ve recommended a one-time boost of Tdap for adults.
To the question of whether adding another dose of pertussis containing vaccine to the adolescent/young adult schedule would reduce cases, Clark showed some models that indicated that boosting at various adolescent and post-adolescent ages would not significantly reduce disease.
Next, Clark addressed the appearance of pertussis strains that lack pertactin, one of the five pertussis vaccine antigens. These pertactin negative strains seem to be increasing in relation to other pertussis strains. In the Washington state epidemic of 2012, about 60% of strains examined were pertactin-negative strains (Pawloski LC et al. 2014). Data from other studies have shown that fully vaccinated people are more likely to be infected with pertactin-negative strains than unvaccinated people. So this may be evidence, heretofore lacking, that the emergence of these strains may in fact be related to pressures applied to the organism from the vaccine.
The next presentation focused on an ethical issue at the heart of vaccination decision making and policy recommendation. This is the question of when there are known adverse events from vaccination, how do we decide that the benefits of the vaccination exceed the risks? The background for this presentation by Umesh D. Parashar, MBBS, of the CDC, was the recall of the first rotavirus vaccination in 1999. This vaccine resulted in a 30-fold increased risk of intussusception, a potentially dangerous intestinal condition, in the week after receipt of the first dose of the vaccine.
The newer vaccines by Merck and GSK went through large clinical trials (>60,000 participants) that did not demonstrate an increased risk for intussusception. Now 55 countries, including low-income countries, are using rotavirus vaccine.
However, the clinical trials for the two new vaccines weren’t large enough to detect any very rare risk of intussusception associated with the newer vaccines. Now we have post-licensure data on the vaccines from Brazil, Mexico, Australia, and the United States. Evidence from Mexico and Brazil indicates an additional risk of intussusception from rotavirus vaccine of 1-2 cases per 100,000 vaccinated children. Data from Australia and the United States demonstrate a similar level of risk.
Despite this risk of intussusception, the benefits of rotavirus vaccine are many. Vaccination has reduced morbidity and mortality from rotavirus everywhere it’s been studied. And this benefit accrues to the non-vaccinated as well: older siblings of vaccinated children have less rotavirus disease, and there has been less disease in adults as well. This effect is evident in the United States and it has also been observed in El Salvador.
Analyzing the data on risk of intussusception and disease reduction has led the WHO vaccine safety committee to conclude that the benefits of rotavirus vaccination outweigh the risks and to extend the recommended age for receipt of the first dose of rotavirus vaccine to 15 weeks because the overall benefit is so positive (see Update on intussusception following rotavirus vaccine administration, 2014 and Rotavirus vaccines and intussusception, 2012).
Sessions continue at the conference all day Tuesday and Wednesday. On Tuesday afternoon, rubella vaccine developer Stanley A. Plotkin, MD, will receive the Dr. Charles Mérieux Award for his lifetime of work on vaccines.