Discussion

Our review confirmed the significant decrease of non–SARS-CoV-2 respiratory viruses, including influenza A, influenza B, RSV, and rhino/enterovirus, during the first year of the COVID-19 pandemic compared to the previous 5 years. Several factors (both community and healthcare related) may help explain the observed decrease in respiratory virus infections (Table 3). Whereas many of the factors listed in Table 3 have been suggested as contributing to the reduction in respiratory infections, quantification of the contribution of any one of these factors is difficult, if not impossible, because most were implemented simultaneously as COVID-19 mitigation strategies. Lee et al ^{Reference Lee, Lee and Song17} analyzed Korean national data to demonstrate that implementation of nationally sponsored COVID-19 mitigation strategies was associated with diminished influenza activity and a truncated influenza season. Olsen et al ^{Reference Olsen, Azziz-Baumgartner and Budd8} noted substantial decreases in detection of influenza in the United States, Australia, Chile, and South Africa. Chan et al ^{Reference Chan, Tambyah, Tee and Somani18} noted that, while influenza A remained suppressed, other respiratory viruses (eg, RSV, enteroviruses/rhinoviruses) returned after schools were reopened in Singapore. Nawrocki et al ^{Reference Nawrocki, Olin and Holdrege19} found a clear association between physical distancing policies and community transmission of 11 non-COVID-19 respiratory viruses.

In our institution, molecularly diagnosed respiratory infections decreased dramatically, despite the personal stress of the pandemic as well as a host of additional pandemic-related factors. Intuitively, these factors might have been thought to facilitate rather than inhibit transmission of viral respiratory pathogens (Table 4). Nonetheless, even in the face of intense institutional COVID-19 activities and increased acuity of inpatients who remained hospitalized, detection of these viral respiratory pathogens dramatically decreased (Figs. 1 and 2; Table 2). Molecularly diagnosed influenza A and influenza B infections decreased to zero during the pandemic.

Table 3.

Nonpharmaceutical Factors Possibly Contributing to Decreased Detection of Respiratory Virus Infections

Note. AGP, aerosol-generating procedure.

Table 4.

Pandemic-Associated Factors That Could Have Facilitated Respiratory Virus Transmission in Healthcare Settings

Note. ICU, intensive care unit.

Fig. 2.

Frequency of detection by virus among NIH Clinical Center patients between January 2015 and March 2021. Observations of substantial statistical differences between year and seasonal periods for each virus are specified in Table 2. *indicates the COVID-19 period.

Our study had several limitations. First the patient population at the NIHCC is not typical, even for most academic centers. The Clinical Center’s patient population is focused in 2 areas—patients who have rare, often heritable chronic diseases (many of whom have disseminated malignancies) and phase 1 and 2 clinical trials. Most of the patient population isimmunosuppressed, placing these patients at high risk for all types of infections, including repeatedly positive viral test results. The inclusion and exclusion criteria described earlier controls for the possibility of repeat positives for the same infection but may also exclude some reinfections incorrectly presumed to be long-term infections.

A second limitation is that milder infections that did not prompt clinicians to order testing may be underrepresented. Because of the pandemic, the threshold for ordering such tests was likely lowered, but the focus may have been on ruling out severe acute respiratory coronavirus virus 2 (SARS-CoV-2). A third limitation is that many aspects of care in our hospital changed dramatically in response to the pandemic. The hospital census declined substantially because only acutely ill patients were hospitalized, elective admissions were curtailed, and elective procedures were postponed. Furthermore, visitors were restricted; only staff essential to patient care were allowed into the hospital; and dedicated COVID-19 care units (including an intensive care unit) were created.

Finally, some investigators have explored whether direct viral–viral interactions may play a role in these findings. ^{Reference Chan, Carolan and Korenkov20–Reference Peng, Shin, Li, Wu and Herrler23} The ecologic aspects of seasonal and pandemic viral infections are areas of active research, but they are currently incompletely defined.

The decrease in the incidence of molecularly diagnosed respiratory viral infections in our hospital during the first 12 months of the COVID-19 pandemic was striking. Numerous factors noted above and in Table 3 likely contributed to this reduction. Many, if not most, published manuscripts suggest the primary reason for such a decrease relates directly to the implementation of COVID-19 mitigation strategies (Table 3). ^{Reference Groves, Piche-Renaud and Peci3,Reference Olsen, Azziz-Baumgartner and Budd8,Reference de Souza Luna, Perosa and Conte13–Reference Wong, Lam and AuYeung16,Reference Cheng, Yu and Liu24–Reference Yeoh, Foley and Minney-Smith37} However, as noted previously, demonstrating the effect of any individual intervention is often challenging. Universal masking represented a major departure from prior infection prevention strategies in our institution that we suspect substantially affected the risk for respiratory virus transmission. A recently published meta-analysis that examined results from 72 studies conclusively demonstrated that several mitigation strategies were associated with a substantial decrease in the risk for transmission of SARS-CoV-2, including mask wearing, handwashing, and physical distancing. ^{Reference Talic, Shah and Wild38} The effectiveness of universal masking (both for personal protection and source control) has been a striking feature of this pandemic.

The colder weather in the late fall and early winter of 2021 may already be heralding the return of non-SARS-COV-2 respiratory infection. Some institutions have observed an increase in RSV and other respiratory virus infections. ^{Reference Li, Yu and Zhang39}

The COVID-19 pandemic continues, with the current δ (delta) variant increasing in prevalence in the upper Midwest and the troubling discovery of the newly detected o (omicron) strain. ⁴⁰ The persistence, and perhaps even escalation, of pandemic COVID-19 cases argues for the consistent application of the effective nonpharmaceutical interventions discussed above. Even when the pandemic subsequently subsides, when influenza or another respiratory virus again emerges, we will advocate for empiric universal masking in our institution.