Viral Outbreaks, Vaccines, and Immunity: a Case Study on the Mumps Outbreak and its Implications for COVID-19

Love Tsai, Life Sciences, July 5, 2020

In the United States, vaccines for the mumps are often given in combination with vaccines for the measles and rubella as the single-dose MMR vaccine. A completed schedule of the MMR vaccine involves two shots and has been long believed to ensure full immunity, but a recent multi-state mumps outbreak starting from a fully vaccinated, asymptomatic carrier has health experts wondering if this vaccine is no longer sufficient.

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News headlines in August 2019 broadcasted a peculiar story with scattered details: a viral disease, asymptomatic carriers, and a multistate outbreak resulting in 61 transmitted cases from one wedding.¹ Nowadays, this may sound like another coronavirus outbreak, but it is actually from a different disease—mumps.

After the establishment of the two-dose MMR vaccine in 1989, mumps was considered nearly eradicated in the United States. But over the past 20 years, mumps has seen a resurgence in the country alongside other locally-transmitted diseases such as measles, mostly due to the growth of the anti-vaccine movement.⁵ This headline, however, was about a special case: the index patient was fully vaccinated, as were many other patients who eventually caught the disease. In other words, this incident shouldn’t have happened at all.

Having acquired the disease from a child who had received the first dose and was awaiting the second one, a 25-year old Nebraska resident went to a wedding showing no symptoms. This event resulted in 31 secondary cases, 27 tertiary cases, and three quaternary cases.¹ Many physicians were surprised to find that 40 of these patients were also fully vaccinated. A questionnaire was sent to all people who may have come into contact with the patient, sick people were isolated, and a vaccination campaign was started and publicized.² Two months after the first case, 327 people received an MMR vaccine dose at the community fire station as a result of this campaign.

The first question arising from this case is the problem of dosages. Most studies show that a two-dose schedule is sufficient protection against the mumps, but this outbreak suggests that a third dose in adulthood may be necessary as well, at least for at-risk persons.¹ In addition, waning childhood immunity is also a problem for many other viral diseases that are treated with vaccines, particularly after the COVID-19 pandemic. The currently recommended schedule for the MMR vaccine confers an 88% risk reduction, so as cities densify and public health evolves, an increased standard may be necessary.² If this is the case, who should get a three-dose schedule, and how will such criteria for selection be determined?

The second question deals more closely with the isolation and quarantine. This particular outbreak occurred in Nebraska where only ill patients were told to isolate, a situation free of the overarching stay-at-home restrictions that many jurisdictions around the world have placed during the COVID-19 pandemic.² As many protests around the country have shown, coronavirus restrictions are heavily unpopular. Thus, with the relatively successful containment of this mumps outbreak, researchers are naturally turning to the implications of epidemics in the 21st century and mandatory quarantine. Questions arise such as: when is such strict isolation necessary? How can the United States implement contract tracing while still respecting individual privacy? Where does personal liberty stop and communal health start?

COVID-19 has a much lower R₀ and serial interval value than those of the mumps. In a susceptible population, the mumps has an R₀ value of 10-12 with a serial interval of 20 days.⁴ That is, from one initial case, 10 – 12 secondary cases can be expected. After the formulation of the vaccine, the United States saw a 99% decrease in mumps cases and it is still a very rare disease today, so much so that outbreaks such as the one in Nebraska draw national attention.² In contrast, the coronavirus’ highest R₀ value has been recorded at 5.7 with a serial interval of 4 – 4.5 days.³ On account of the differences in these numbers, in addition to their inherent biological differences, the ways and rate at which the modern world defeated the mumps may not be completely comparable to the path that COVID-19 will follow. Nonetheless, just as polio, hepatitis-B, and tetanus have been eliminated in the United States, continuous inquiry into both the MMR vaccine and a potential coronavirus vaccine can render such a future where these two diseases are a thing of the past.

Bibliography

1. Donahue, M., Hendrickson, B., Julian, D., Hill, N., Rother, J., & Koirala, S. et al. (2020). Multistate Mumps Outbreak Originating from Asymptomatic Transmission at a Nebraska Wedding — Six States, August–October 2019. MMWR. Morbidity And Mortality Weekly Report, 69(22), 666-669. https://doi.org/10.15585/mmwr.mm6922a2
2. Ekemini Hogan, U. (2020). A Mumps Outbreak among Fully Vaccinated People. Scientific American. Retrieved 6 July 2020, from https://www.scientificamerican.com/article/a-mumps-outbreak-among-fully-vaccinated-people/.
3. Fisher, M. (2020). R0, the Messy Metric That May Soon Shape Our Lives, Explained. Nytimes.com. Retrieved 6 July 2020, from https://www.nytimes.com/2020/04/23/world/europe/coronavirus-R0-explainer.html.
4. Gupta, R., Best, J., & MacMahon, E. (2005). Mumps and the UK epidemic 2005. BMJ, 330(7500), 1132-1135. https://doi.org/10.1136/bmj.330.7500.1132
5. Hoffman, J. (2020). How Anti-Vaccine Sentiment Took Hold in the United States. Nytimes.com. Retrieved 6 July 2020, from https://www.nytimes.com/2019/09/23/health/anti-vaccination-movement-us.html.
6. Mumps | Cases and Outbreaks | CDC. Cdc.gov. (2020). Retrieved 6 July 2020, from https://www.cdc.gov/mumps/outbreaks.html.