In this column in September 2020, you read how common respiratory viruses’ seasons are usually so predictable, each virus arising, peaking, and then dying out in a predictable virus parade (Figure 1).1 Well, the predictable virus seasonal pattern was lost in 2020. Since March of 2020, it is striking how little activity was detected for the usual seasonal viruses in Kansas City after mid-March 2020 (Figure 2).2 So, my concern in September 2020 for possible rampant coinfections of common viruses with or in tandem with SARS-CoV-2 did not pan out. That said, the seasons for non–SARS-CoV-2 viruses did change; I just didn’t expect they would nearly disappear.
Figure 1
Dr. Christoper Harrison
The 2020 winter-spring. In the first quarter (the last part of the overall 2019-2020 respiratory viral season), viral detections were chugging along as usual up to mid-March (Figure 2); influenza, respiratory syncytial virus (RSV), and rhinovirus were the big players.
Influenza. In most years, influenza type B leads off and is quickly replaced by type A only to see B reemerge to end influenza season in March-April. In early 2020, both influenza type A and influenza type B cocirculated nearly equally, but both dropped like a rock in mid-March (Figure 2).2 Neither type has been seen since with the exception of sporadic detections – perhaps being false positives.
RSV. In the usual year in temperate mid-latitudes of the northern hemisphere, RSV season usually starts in early December, peaks in January-March, and declines gradually until the end of RSV season in April (Figure 1). In southern latitudes, RSV is less seasonal, being present most of the year, but peaking in “winter” months.3 But in 2020, RSV also disappeared in mid-March and has yet to reappear.
Other viruses. Small bumps in detection of parainfluenza of varying types usually frame influenza season, one B bump in early autumn and another in April-May. In most years, human metapneumovirus is detected on and off, with worse years at 2- to 3-year intervals. Adenovirus occurs year-round with bumps as children get back to school in autumn. Yet in 2020, almost no parainfluenza, adenovirus, common coronaviruses, or human metapneumovirus were detected in either spring or autumn. This was supposed to be a banner summer-autumn for EV-D68 – but almost none was detected. Interestingly, the cockroach of viruses, rhinovirus, has its usual year (Figure 2).
Figure 2
Dr. Christopher Harrison
What happened? Intense social mitigation interventions, including social distancing and closing daycares and schools, were likely major factors.4 For influenza, vaccine may have helped but uptake was not remarkably better than most prior years. There may have been “viral competition,”where a new or highly transmissible virus outcompetes less-transmissible viruses with lower affinity for respiratory receptors.5,6 Note that SARS-CoV-2 has very high affinity for the ACE2 receptor and has been highly prevalent. So, SARS-CoV-2 could fit the theoretical mold for a virus that outcompetes others.
Does it matter for the future? Blunted 2019-2020 and nearly absent 2020-2021 respiratory virus season may have set the stage for intense 2021-2022 rebounds for the non–SARS-CoV-2 viruses. We now have two whole and one partial birth cohort with no experience with seasonal respiratory viruses, including EV-D68 (and nonrespiratory viruses too – like norovirus, parechovirus, and other enteroviruses). Most viruses have particularly bad seasons every 2-3 years, thought to be caused by increasing accumulation of susceptible individuals in consecutive birth cohorts until a critical mass of susceptible individuals is achieved. The excess in susceptible individuals means that each contagious case is likely to expose one or more susceptible individuals, enhancing transmission and infection numbers in an ever-extending ripple effect. We have never had this many children aged under 3 years with no immunity to influenza, RSV, etc. So unless mother nature is kind (when has that happened lately?), expect rebound years for seasonal viruses as children return to daycare/schools and as social mitigation becomes less necessary in the waning pandemic.
Dr. Christopher J. Harrison
Options? If you ramped up telehealth visits for the pandemic, that may be a saving grace, i.e., more efficiency so more “visits” can be completed per day, and less potential contact in reception rooms between well and ill children. And if there was ever a time to really intensify efforts to immunize all our pediatric patients, the next two seasons are just that. Adding a bit of a warning to families with young children also seems warranted. If they understand that, while 2021-2022 will be better for SARS-CoV-2, it is likely going to be worse for the other viruses.
Dr. Harrison is professor of pediatrics and pediatric infectious diseases at Children’s Mercy Hospitals and Clinics, Kansas City, Mo. He said he had no relevant financial disclosures. Email him at pdnews@mdedge.com.
References
1. Harrison CJ. 2020-2021 respiratory viral season: Onset, presentations, and testing likely to differ in pandemic, Pediatric News: September 17, 2020.
2. Olsen SJ et al. MMWR Morb Mortal Wkly Rep. 2020;69:1305-9.
3. Respiratory Syncytial Virus Surveillance. http://www.floridahealth.gov/diseases-and-conditions/respiratory-syncytial-virus/_documents/2021-w4-rsv-summary.pdf
4. Baker RE et al. PNAS. Dec 2020 117;(48):30547-53.
5. Sema Nickbakhsh et al. PNAS. Dec 2019 116;(52):27142-50.
6. Kirsten M et al. PNAS. Mar 2020 117;(13):6987.