Which Adenoviruses Circulate in the U.S.? Type Reporting Fell After COVID-19

In brief

• Human adenoviruses (HAdVs) cause a wide range of respiratory and other illnesses, and outbreaks in congregate settings.
• CDC's National Adenovirus Type Reporting System (NATRS) — passive and voluntary, collecting data since 2003 and formally launched in 2014 — tracks which types circulate.
• In 2017–2023, six types (1–4, 7 and 14) made up 88.3% of typed specimens, and 17.0% were outbreak-related.
• Reporting fell during and after the COVID-19 pandemic, although adenoviruses kept circulating. More labs need to type and report them.

Background

Adenoviruses fall into seven species (A–G) and about 100 types. Besides respiratory illness they cause gastroenteritis and conjunctivitis. Most infections are mild or silent, but severe illness occurs — sporadically in healthy people, more often in those with weakened immunity — and outbreaks have hit college campuses, hospitals and military training sites. A vaccine against types 4 and 7, which have caused severe outbreaks, is available only to the military.

Because typing rarely changes treatment, few labs type adenoviruses routinely; specimens are typed mostly during outbreak investigations, so outbreak specimens may be overrepresented.

What was reported

Six laboratories — CDC's respiratory virus lab, four state public health labs and one Department of Defense lab — reported 2,909 specimens from patients in 30 states. After removing records missing type or date and duplicates, 2,241 remained.

Figure 1. Where the typed specimens came from, by patient's state and reporting laboratory, 2017–2023. CDC, MMWR.

• Specimens: 82.3% respiratory, 6.4% stool.
• Reports per year: rose from 389 (2017) to 562 (2019), collapsed to 58 (2020) and 103 (2021), then 356 (2022) and 273 (2023) — below the prepandemic average of 484.
• Ages: mostly 0–4 years (45.0%), 18–29 (27.5%) and 5–17 (16.0%); adults 30 and older only 4.2%.

The mix changed over time: types 4, 3 and 7 led in 2017–2019; types 2, 1, 41 and 4 in 2020–2022; and type 3 (68.9%) dominated 2023.

Figure 2. Specimen types (A) and adenovirus types (B) reported, by year of collection. CDC, MMWR.

Outbreaks: 381 (17.0%) specimens were outbreak-related. In 2017–2019, 18.4% were, most often types 7, 4 and 14. In 2020–2021, 10.6%, including a nationwide investigation of pediatric hepatitis suspected to be linked to type 41; in 2022, 27.2%, mostly types 4 and 41; in 2023, none.

What it means

The same six types accounted for 86% of specimens in 2003–2016, suggesting that vaccines and treatments covering certain types could curb community and outbreak illness. Outbreaks shaped the trends: a type 7 outbreak among unvaccinated military trainees in Virginia in 2017, a type 4 outbreak at a military academy in Connecticut in 2019, and type 4 and 7 outbreaks at colleges in several states in 2018–2022.

Pandemic measures in 2020 disrupted most respiratory viruses, and adenovirus activity reported to the National Respiratory and Enteric Virus Surveillance System dipped — but it was back to normal by spring 2021. So the fall in NATRS reports most likely reflects less typing and reporting, not less virus.

Limitations: few labs can type adenoviruses, so the data aren't geographically representative; incomplete records were excluded; there is little clinical detail; and typing favors severe cases and outbreaks.

Labs are encouraged to type positive specimens, or send them for typing, and report to NATRS; the growth of genome sequencing since the pandemic could help.

Sources

Based on Fatima Abdirizak and others, "Surveillance of Human Adenovirus Types and the Impact of the COVID-19 Pandemic on Reporting — United States, 2017–2023," MMWR Morbidity and Mortality Weekly Report, volume 73, Centers for Disease Control and Prevention; a work of the United States government in the public domain.