Outbreaks, Alerts and Hot Topics

More Than a Decade of Mumps Data in Children and an Influenza Outbreak on a College Campus

Column Editor: Angela Myers, MD, MPH | Director, Division of Infectious Diseases | Professor of Pediatrics, UMKC School of Medicine | Medical Editor, The Link Newsletter

I want to talk about something other than COVID-19 this month, as I am sure we could all use a break from it.

Despite the high level of routine childhood vaccination across our country, we have seen sporadic cases as well as intermittent outbreaks of mumps in schools since 2006. I distinctly remember the 2006 outbreak across eight Midwestern states resulting in >6,500 cases during my second year of ID fellowship.¹ As a point of reference, we had approximately 200-300 mumps cases in the U.S. annually from 2000 to 2005.² A recent article was published in Pediatrics evaluating mumps cases from 2007 to 2019.³ The authors identified more than 9,000 cases of mumps across this time span with a median of one-third of cases occurring in those younger than 18 years of age. Most (81%-94%) children and teenagers had received at least one dose of measles-mumps-rubella (MMR) vaccine with 74% of 1- to 4-year-olds having ≥1 dose and 86% of 5- to 17-year-olds having ≥2 doses. While it may seem odd that the majority of those infected were fully vaccinated, it is well known that the mumps virus is the least immunogenic of the three viruses included in the MMR vaccine. The mumps component prevents disease in 88% following two doses of vaccine and 78% after one dose.⁴ This compares to measles and rubella, which is 97% effective after two doses and one dose respectively. Additionally, immunity to mumps may wane over time, leaving vaccinated teenagers and young adults more susceptible to infection compared to younger, fully vaccinated children, who are more likely to still have adequate protection.⁵

In this study, the peak years were 2016, with 6,366 cases, and 2017, with 6,109 cases (both near the previous peak in 2006).² In these two peak years, most cases occurred in 11- to 17-year-olds (40-45 per million), followed by 5- to 10-year-olds (20-35 per million). One percent of patients had a complication from mumps infection. The table below provides the breakdown by complication and vaccine status. There were no mumps-related deaths noted in this study.

About two-thirds of mumps cases over this time span were outbreak-associated, but this finding was skewed by the two peak years (93% outbreak-associated in 2016 and 85% in 2017). Therefore, in most years, the largest proportion of cases were sporadic. Additionally, every state reported mumps cases over the course of the study period with most states reporting both adult and pediatric cases in each year. Finally, travel history was available for 74% of cases, and only 2% were related to international travel.³

What do these data tell us?

1. We are doing well, but we could do better. We need to continue to work on optimizing routine MMR vaccination of children.
2. Complications were relatively rare.  
3. While mumps occurred more frequently in older kids/teens in peak years, which is typical of outbreaks, mumps remains in our differential for younger (even immunized) children.
4. Mumps still occurs each year in the U.S., is not generally related to travel, and sporadic cases are the norm in non-peak years.

Table. Complications from Mumps

* Age of each case instead of median

In early November, the University of Michigan notified the public health department of a rapid rise in influenza A H3N2 cases. All symptomatic people were tested for influenza, respiratory syncytial virus (RSV), and COVID-19. From early October through the week before Thanksgiving, 745 (23.9%) cases of influenza A cases were identified, along with 137 (4.4.%) COVID-19 and 84 (2.7%) RSV.⁶ Nearly all the influenza cases occurred after Nov. 1; slightly more than one-fourth had received the 2021-22 seasonal influenza vaccine (the same rate as those with symptoms but testing negative for influenza). There was only one reported hospitalization. These data may be an early indication that the H3N2 strain included in this year’s vaccine may not provide robust protection against mild disease. While we cannot make conclusions from these data yet, the vaccine has historically performed less well against H3N2 compared to the other serotypes (H1N1 and B) included in the vaccine. This is because inter-seasonal mutations are more frequent with H3N2 viruses. Thus, the circulating strain can evade immunity induced by the seasonal vaccine H3N2 strain, which was chosen based on best estimates from the dominant strain that had circulated during the Southern Hemisphere’s winter. These breakthrough cases are preliminary evidence of a mismatch between the vaccine strain (3C.2a1b.2a.1) and the circulating Michigan strain (3C.2a1b.2a.2), which may become this winter’s circulating H3N2 strain in the U.S.

While we don’t yet know what this viral respiratory season will look like overall, influenza cases have been reported in our community. We also saw our first case of influenza at Children’s Mercy during the first week of December and have started routine influenza testing. As a reminder, all children with influenza-like-illness should be tested for both influenza and COVID-19, given the opportunity for treating influenza within the first 48 hours of illness and the public health implications of the SARS-CoV-2 detections. These data also remind us to continue to recommend influenza vaccination (as I know you do). While we may not prevent mild H3N2 disease, there are still three other strains in the vaccine, and we know that the vaccine can prevent hospitalization and death. We can also help prevent co-infections, which often lead to more severe disease. And finally, immunizing healthy children can help prevent influenza and its complications in high-risk populations who may not respond well to influenza vaccine.

I hope you all have a happy and healthy 2021 holiday season, and I look forward to what 2022 will bring us. 

References:

1. Dayan GH, Quinlisk MP, Parker AA, et al. Recent resurgence of mumps in the United States. N Engl J Med. 2008;358(15):1580-1589. 
2. Accessed December 4, 2021. https://www.cdc.gov/mumps/outbreaks.html
3. Shepersky L, Marin M, Zhang J, Pham H, Marlow MA. Mumps in vaccinated children and adolescents: 2007-2019. Pediatrics. 2021;148(6):e2021051873.
4. Accessed December 6, 2021. https://www.cdc.gov/vaccines/vpd/mmr/public/index.html
5. Lewnard JA, Grad YH. Vaccine waning and mumps re-emergence in the United States. Sci Transl Med. 2018;10(433):eaao5945.
6. Delahoy MJ, Mortenson L, Bauman L, et al. Influenza A(H3N2) outbreak on a university campus — Michigan, October–November 2021. MMWR Morb Mortal Wkly Rep. Published online December 3, 2021.