Study Raises Concerns about General Population’s Herd Immunity
By Annette M. Boyle
BETHESDA, MD — Lower immunization rates have increased the number of U.S. outbreaks of vaccine-preventable diseases, even of those considered eradicated in this country, such as measles.
The question for military medicine is whether rates have dropped low enough to compromise herd immunity among new servicemembers. A recent study of Air Force recruits found too low seroprevalence for measles and mumps and questionable rates for rubella.
“While this data provide a nice snapshot of immunity levels in this age group, they don’t reflect immune status of our military population,” said Air Force Lt. Col. Paul E. Lewis, III, MD, MPH. “All new recruits found to be seronegative are immunized on entry to the military.”
For other groups, however, the results might be more problematic, Lewis pointed out, explaining, “If these seroprevalence rates are generalizable to their counterparts entering college and the workforce across the U.S., outbreaks of vaccine-preventable disease can be anticipated to continue.”
The Air Force tests recruits entering basic training for immunity to measles, mumps and rubella, as well as three other diseases, partly because close quarters, fatigue and stress of military life might increase susceptibility. Recruits receive vaccines for any of the six diseases for which they show inadequate immunity.
Researchers at the Uniformed Services University of the Health Sciences in Bethesda, MD, the Defense Health Agency and Joint Base Lackland-San Antonio studied 32,502 recruits who entered basic training at the base from April 2013 until April 2014. Their study, published in the American Journal of Preventive Medicine, found that seroprevalence rates for measles and mumps fell below minimum levels required for herd immunity at 81.6% and 80.3%, respectively.1
Seroprevalence rates of at least 85% are needed to provide herd immunity for measles and mumps, though optimal rates may be as high as 86% for measles and 90% for mumps. For rubella, herd immunity requires seroprevalence rates between 77% and 86%. The rate of seropositivity for rubella for incoming servicemembers exceeded the minimum estimate, at 82.1%, but fell below the upper range of the necessary levels for herd immunity.
Variations in Immunity
Herd immunity levels vary by disease, based on how easily the virus is transmitted from person to person. Exactly what level provides protection from outbreaks depends on a number of other factors, as well.
“Estimates vary as to the level required to prevent an outbreak and the sensitivity and specificity of the test to measure seropositivity to each disease also varies,” Lewis, the lead author, told U.S. Medicine. “The upper and lower thresholds take both these factors into account.”
The seropositive levels in the recent study reflect a significant decline from an analysis of recruits entering the U.S. military from 2000 to 2004. That cohort study found seropositivity rates of 84.6%, 89.5% and 93.2% for measles, mumps and rubella, respectively. 2
The National Health and Nutrition Examination Survey conducted from 1999 to 2004 found even higher rates of detectable immunity among individuals born between 1977 and 1986 — 96% for measles, 90% for mumps and 89% for rubella.3,4,5
Seroprevalence rates for women ran 4%-6% higher than men for all three viruses. The authors noted that in the women studied, the higher rates “may be due to increased immune response in women, as noted after influenza vaccination.” In the general population, similar results may be expected because of the emphasis on rubella vaccination of women after pregnancy, although that factor is unlikely to explain the difference in the young women tested upon entering the service.
Because the U.S. uses a trivalent vaccine for measles, mumps and rubella (MMR), the DoD recommends presuming mumps immunity in those who test positive for measles and rubella immunity.
In the 2000-2004 study of recruits, 92.8% of those demonstrating measles and rubella immunity also were seropositive for mumps, leading the authors of that study to conclude that “these findings support the policy of targeting MMR immunization based upon measles and rubella serology alone.”
The recent study challenges continued assumption of seroconcordance between mumps and measles and rubella. Only 87.7% of recruits who had positive titers for measles and rubella were also seropositive for mumps.
Lewis and colleagues noted that “had the U.S. Air Force followed current DoD immunization policy and presumed mumps immunity based on measles and rubella titer status, more than 2,800 new recruits would not have been vaccinated against the mumps virus, perhaps leaving the population vulnerable to mumps outbreak with sub-herd immunity vaccination levels.” As a result, they recommended that the DoD consider testing recruits for immunity to all three diseases and vaccinating all individuals who indicate susceptibility to any of the viruses.
- Lewis PE, Burnett DG, Costello AA, Olsen CH, Tchandja JN, Webber BJ. Measles, Mumps, and Rubella Titers in Air Force Recruits: Below Herd Immunity Thresholds? Am J Prev Med. 2015 Jun 25.
- Eick, AA, Hu, Z, Wang, Z, and Nevin, RL. Incidence of mumps and immunity to measles, mumps and rubella among U.S. military recruits, 2000-2004. Vaccine. 2008; 26: 494–501
- McQuillan, GM, Kruszon-Moran, D, Hyde, TB, Forghani, B, Bellini, W, and Dayan, GH. Seroprevalence of measles antibody in the U.S. population, 1999-2004. J Infect Dis. 2007; 196: 1459–1464.
- Kutty, PK, Kruszon-Moran, DM, Dayan, GH, et al. Seroprevalence of antibody to mumps virus in the U.S. population, 1999-2004. J Infect Dis. 2010; 202: 667–674.
- Hyde, TB, Kruszon-Moran, D, McQuillan, GM, et al. Rubella immunity levels in the United States population: Has the threshold of viral elimination been reached? Clin Infect Dis. 2006; 43: S146–S150.