Respiratory syncytial virus (RSV) detections have been reportable in Arizona since 2004, through the state's passive infectious disease surveillance. All clinical laboratories must report positive results, some health care providers also report, and both inpatient and outpatient facilities are covered. A case is a laboratory-positive result during the RSV season, which in Arizona runs from October to September.
A shift in who tested positive
In the 2016–17 season, the Arizona Department of Health Services noticed a change in the ages of patients with reported RSV:
- From 2009–10 through 2012–13, more than 90% of each season's cases were in children under 5.
- By 2016–17, children under 5 made up 78%, while adults 65 and older had risen from 1% of cases in 2009–10 to 11%.
At the same time, PCR (polymerase chain reaction) testing for RSV had been increasing across the United States, and antigen testing decreasing. The department asked: had RSV itself changed, or had testing?
What the data showed
Reported tests were grouped as rapid antigen, PCR or other (culture or immunofluorescence assays); between 1% (2015–16) and 11% (2013–14) could not be categorized.
| Season | Reported cases | Under 5 | 65 and older | PCR-positive, all ages | PCR-positive, under 5 | PCR-positive, 65+ |
|---|---|---|---|---|---|---|
| 2013–14 | 2,735 | 89% | 4% | 21% | 18% | 58% |
| 2014–15 | 5,152 | 84% | 6% | 44% | 38% | 77% |
| 2015–16 | 4,433 | 82% | 9% | 49% | 41% | 87% |
| 2016–17 | 5,434 | 78% | 11% | 53% | 45% | 88% |
- The share of cases in children under 5 fell every season, and the share in adults 65+ rose every season (a significant trend).
- The share of cases found by PCR rose 152%, from 21% to 53%.
- PCR use rose in every age group, but the largest increase in reported cases was among people 65 and older — and in every season a much larger share of older adults' cases than young children's were PCR detections.
Why it matters
The rise in PCR detections since 2013 matches the shift in age. RSV has historically been diagnosed mostly in young children, but awareness of infection in older adults has grown, which may show up as more testing of that age group. Because PCR use differs by age, the change in who tests positive may come from testing practice rather than from the disease — especially if older adults, who otherwise might not have been tested for RSV, are increasingly given PCR-based respiratory viral panels. Antigen tests are also less sensitive in older people, which may lead providers to order PCR for them instead.
Limitations
- The surveillance records only positive results, so a real age-related change in disease cannot be ruled out.
- The share of results with a known test type has grown in recent seasons, perhaps because electronic laboratory reporting records test types better than handwritten reports or manual entry.
Future analyses will add testing data that include negative as well as positive results. As new tools to diagnose and prevent RSV arrive, understanding such changes in surveillance data matters.
Sources
Based on Bridge R, Brady S, Erhart LM, Komatsu K, "Notes from the Field: Age Distribution of Patients with Laboratory-Detected Respiratory Syncytial Virus — Arizona, 2013–2017," MMWR Morbidity and Mortality Weekly Report, volume 68, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's text says about 3,000–5,000 cases were reported each season; its table, used here, gives 2,735 to 5,434.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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