Before a vaccine arrived in 2006, rotavirus was the leading cause of severe gastroenteritis — vomiting and diarrhea — in U.S. children. To measure the vaccine’s long-term effect, CDC analyzed laboratory test results reported to its National Respiratory and Enteric Virus Surveillance System (NREVSS) for the prevaccine (2000–2006) and postvaccine (2007–2018) periods.
A rotavirus “season” was counted from the first to the last of two consecutive weeks in which at least 10% of tests were positive. Historically, rotavirus struck in winter and spring, starting in December–January and ending in April–May.
Far less rotavirus
In the 23 laboratories that reported continuously throughout 2000–2018:
| Annual median | Prevaccine | Postvaccine | Change |
|---|---|---|---|
| Rotavirus tests | 10,845 | 7,357 | about one-third fewer |
| Positive tests | 2,778 | 411 | about 85% fewer |
| Share of tests positive | 25.6% | 6.1% | 76% lower |
Rotavirus tests, positive tests and percent positivity, 23 continuously reporting laboratories, 2000–2018. Image from CDC’s report.
Across all reporting laboratories, the postvaccine seasons were also smaller, later and shorter:
- Peak weekly positivity fell to a median of 14.0% (range 4.8%–27.3%).
- The season began later in the year.
- Its median length fell from 26 weeks (range 23–27) to 9 weeks (range 0–18).
Every postvaccine season from 2007–08 to 2017–18 stayed below the 2000–2006 baseline.
A new two-year rhythm
After the vaccine, rotavirus settled into a biennial pattern, starting in the 2008–09 season: low years alternating with high years.
| Low-activity years | High-activity years | |
|---|---|---|
| Season | never reached the 10% threshold | median 17 weeks (range 10–18) |
| Peak positivity (median) | 8.7% (range 4.8–12.2) | 25.1% (range 21.7–27.3) |
| Peak timing | later, weeks 17–22 | earlier, weeks 11–13 |
Rotavirus season length and peak activity, prevaccine (2000–2006) and postvaccine (2008–2018) years, by even and odd season. Image from CDC’s report.
The likely reason is coverage. Completed rotavirus vaccination has plateaued at approximately 70% — lower than for other childhood vaccines, partly because there is little room to catch up: the first dose must be given by 15 weeks of age and the series finished by 8 months. Unvaccinated children may build up during low years, fueling an outbreak the following season. Countries that quickly reached and held 90%–95% coverage, such as the United Kingdom, have seen a steady decline without the two-year cycle.
Is it the vaccine, or the tests?
Testing changed over the period. Laboratories reporting enzyme immunoassay (EIA) tests fell from 223 (29,198 tests, 2008–09) to 109 (14,737 tests, 2017–18), while those reporting PCR tests rose from 19 (16,490 tests, 2014–15) to 80 (87,775 tests, 2017–18). Shifts toward PCR multi-pathogen panels may explain some of the drop in test counts. But the steep fall in the share of tests that were positive — which changes in testing affect less — points to the vaccine.
Improving coverage and on-time vaccination could extend its benefits further, and watching future seasons will show whether the two-year cycle fades.
Sources
Based on Hallowell BD, Parashar UD, Curns A, DeGroote NP, Tate JE, "Trends in the Laboratory Detection of Rotavirus Before and After Implementation of Routine Rotavirus Vaccination — United States, 2000–2018," MMWR Vol. 68, No. 24, CDC; a work of the United States government in the public domain. The report gives the prevaccine median peak positivity as 43.1% with a range of 43.8%–56.3%, which cannot both be right, so that figure is left out.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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