A Wedding, an Unsuspected Case and a Six-State Mumps Outbreak, 2019

This report describes an outbreak in 2019.

Since 2006, most U.S. mumps cases have occurred in people who received two doses of measles-mumps-rubella (MMR) vaccine, and mumps is most contagious just before and as the swelling of the salivary glands (parotitis) begins. In August 2019, 30 guests at a Nebraska wedding caught mumps from a single fully vaccinated guest who had no symptoms yet. The chain of infection spread through three more generations and six states.

Finding the outbreak

On August 26, 2019, a South Dakota hospital told the Nebraska Department of Health and Human Services (NDHHS) of three suspected mumps cases in people who had attended an August 3 wedding in Nebraska. The bride supplied a guest list of 176 families, about 325 people from 14 states, and named 25 guests she believed were ill, including one who had fallen sick less than 24 hours after the wedding, 15 days before anyone else.

NDHHS alerted public health partners nationwide on August 28 that the outbreak could reach multiple states, sent health alerts to providers in Nebraska and South Dakota the next day, and issued a media statement. It emailed and mailed an online questionnaire to all 176 families, asked providers, health departments and laboratories to report suspected cases, interviewed patients by phone, advised them to follow mumps isolation precautions, and checked vaccination histories against state registries. Of the roughly 325 guests, 148 (46%) answered the questionnaire.

The index patient

The first case was a 25-year-old Nebraska woman who worked caring for children. Beginning July 25, she spent six days in close contact with a sick 1-year-old just back from a family vacation in Florida and Antigua. The child had received a first, on-schedule MMR dose in June, developed a high fever and kept pulling at an ear, and was seen three times and diagnosed with a viral illness.

She attended the wedding on August 3, nine days after her first exposure, and socialized extensively, sharing drinks and dancing. The next day her left ear and jaw became tender, and on August 5 her parotitis began. She sought care on August 9 and was treated with corticosteroids, but because she was never tested, she was classified as a probable case. She had received both recommended MMR doses, as her registry record confirmed. Neither the child nor the child's family attended the wedding, she had no contact with other guests beforehand, and the ceremony did not include drinking wine from a shared cup.

Generations of cases

Community A is a northeastern Nebraska town of about 1,400 people.

• Secondary cases: 30 were wedding guests, an attack rate of at least 9%, and one was exposed to the index patient elsewhere. Half of the 30 guests had received two MMR doses. Three with no likely exposure other than the wedding became ill 26 to 29 days after it, longer than mumps' usual 12- to 25-day incubation period.
• Tertiary cases: six were linked to secondary cases, 18 to various events in community A, and three were community A residents with no known links.
• Quaternary cases: all three lived in community A and were linked to a tertiary case.

In all there were 62 cases, 39 confirmed and 23 probable. Of these, 54 (87%) were Nebraska residents, 34 of them in community A, and eight lived in South Dakota, Idaho, Minnesota, North Dakota and Wyoming. The median age was 35 (range 6–59), 41 (66%) had received at least two MMR doses, and 37 (60%) were male. There were no serious complications or hospitalizations. Four virus samples from Nebraska patients were tested; three were genotype G and one was indeterminate.

Stopping it

Because the wedding was a single exposure and the guests were widely scattered, officials at first held off on a community vaccination campaign. When 63% of the tertiary cases turned up in community A, they escalated. Flyers went to the chamber of commerce, schools, city and county offices, local radio, television and cable, and the health department's Facebook page and websites. Thirty health officials and volunteers ran a clinic organized under the National Incident Management System. Adults aged 19 to 62 who lived or worked in community A, had no medical reason to avoid the vaccine, and had not had mumps or a mumps vaccine in the previous six months were eligible. On October 3, 327 people, 47% of an estimated 700 in the target population, received an MMR dose at the town's fire station.

As of December 1, 2019, no more cases had been found in community A or linked to the outbreak anywhere else. Cases had started to fall before the campaign, so its exact contribution is hard to judge.

Why it happened

Mumps immunity from childhood vaccination can wane by early adulthood, and with two thirds of patients having had at least two doses and a median age of 35, waning immunity likely played a role. Genotype G is common in U.S. cases and outbreaks, so the virus itself was probably not unusual. What mattered was timing and setting: the wedding came just before the index patient's symptoms began, when mumps is most contagious, and it was a crowded event full of close contact, ideal for droplet spread. The authors conclude that mumps can spread widely from someone without symptoms in the right setting, that a community MMR campaign is worth considering, and that early, regular communication among local, state and national health authorities, providers and community leaders was crucial.

Sources

• Donahue M, Hendrickson B, Julian D, et al. Multistate Mumps Outbreak Originating from Asymptomatic Transmission at a Nebraska Wedding — Six States, August–October 2019. MMWR Morb Mortal Wkly Rep 69(22). https://www.cdc.gov/mmwr/volumes/69/wr/mm6922a2.htm
• The report's discussion says the index patient's parotitis began the day after the wedding, while its case description dates jaw tenderness to that day and parotitis to August 5; this page follows the case description.
• The report was led by the Nebraska Department of Health and Human Services with CDC and state partners; its figure is not reproduced here.