#15,462
One of the intriguing side effects of the COVID-19 pandemic has been the concurrent and dramatic drop in influenza activity around the globe. Within weeks of COVID's emergence - and the imposition of social distancing and NPI measures - influenza activity in the northern hemisphere plummeted, and never took off in the southern hemisphere during their winter (see below).
While some of this drop-off was initially attributed to a redirection of reporting and surveillance assets due to COVID-19, subsequent testing has confirmed that influenza has been greatly suppressed globally for the past 6 months.
The most recent WHO Influenza update (below) shows a blank flu map, and indicates only 34 positive flu samples were received by their laboratories over a two-week period in August (a 120-fold decrease over the same time period a year ago).
- The current influenza surveillance data should be interpreted with caution as the ongoing COVID-19 pandemic have influenced to varying extents health seeking behaviours, staffing/routines in sentinel sites, as well as testing priorities and capacities in Member States. The various hygiene and physical distancing measures implemented by Member States to reduce SARS-CoV-2 virus transmission have likely played a role in reducing influenza virus transmission.
- Globally, influenza activity was reported at lower levels than expected for this time of the year. In the temperate zones of the southern hemisphere, the influenza season has not started. Despite continued or even increased testing for influenza in some countries in the southern hemisphere, very few influenza detections were reported.
- In the temperate zone of the northern hemisphere, influenza activity remained below inter-seasonal levels.
- In the Caribbean and Central American countries, no influenza detections were reported. Severe acute respiratory infection (SARI) activity, likely due to COVID-19, appeared to decrease in some reporting countries.
- In tropical South America, tropical Africa and Southern Asia there were sporadic or no influenza detections across reporting countries.
- In South East Asia, influenza A(H3N2) virus detections were reported in Cambodia.
- Worldwide, of the very low numbers of detections reported, seasonal influenza A viruses accounted for the majority of detections.
National Influenza Centres (NICs) and other national influenza laboratories from 52 countries, areas or territories reported data to FluNet for the time period from 17 August 2020 to 30 August 2020 (data as of 2020-09-11 02:38:29 UTC). The WHO GISRS laboratories tested more than 1450681 specimens during that time period: 34 were positive for influenza viruses, of which 19 (55.9%) were typed as influenza A and 15 (44.1%) as influenza B. Of the sub-typed influenza A viruses, 0 (0%) were influenza A(H1N1)pdm09 and 11 (100%) were influenza A(H3N2). Of the characterized B viruses, 3 (37.5%) belonged to the B-Yamagata lineage and 5 (62.5%) to the B-Victoria lineage.
WHO encourages the testing of routine influenza surveillance samples from sentinel and non-sentinel sources for influenza and SARS-CoV-2 virus where resources are available and report this information, separate data from sentinel and non-sentinel sites if possible, to established regional and global platforms (See the Operational considerations for COVID-19 surveillance using GISRS guidance)
While ostensibly a `good thing', this reduction in flu activity around the world could have unexpected impacts (see When Epidemic Viruses Collide). For better or worse, the evolutionary trajectory of seasonal influenza viruses have been disrupted by our COVID-19 pandemic, and potentially changed as a result.
Case in point - next week, the WHO is scheduled to convene an expert panel to select the vaccine components for next year's Southern Hemisphere flu vaccine. With so little flu data to go on, predicting next year's dominant flu strains will be even more daunting than usual.
While there are fears of a `twindemic' this winter of seasonal flu and COVID-19, it is possible that influenza will remain suppressed this winter as well. We are literally in uncharted territory, as we've never dealt with a flu season and a coronavirus pandemic before.
Yesterday's MMWR delves deeper into this decreased global influenza activity, and suggests that while flu activity may be suppressed or delayed this winter, it will still be important to get the flu vaccine this fall.
Weekly / September 18, 2020 / 69(37);1305–1309
Sonja J. Olsen, PhD1; Eduardo Azziz-Baumgartner, MD1; Alicia P. Budd, MPH1; Lynnette Brammer, MPH1; Sheena Sullivan, PhD2; Rodrigo Fasce Pineda, MS3; Cheryl Cohen, MD4,5; Alicia M. Fry, MD1 (View author affiliations)
Summary
What is already known about this topic?
Influenza activity is currently low in the United States and globally.
What is added by this report?
Following widespread adoption of community mitigation measures to reduce transmission of SARS-CoV-2, the virus that causes COVID-19, the percentage of U.S. respiratory specimens submitted for influenza testing that tested positive decreased from >20% to 2.3% and has remained at historically low interseasonal levels (0.2% versus 1–2%). Data from Southern Hemisphere countries also indicate little influenza activity.
What are the implications for public health practice?
Interventions aimed against SARS-CoV-2 transmission, plus influenza vaccination, could substantially reduce influenza incidence and impact in the 2020–21 Northern Hemisphere season. Some mitigation measures might have a role in reducing transmission in future influenza seasons.
(SNIP)
Discussion
In the United States, influenza virus circulation declined sharply within 2 weeks of the COVID-19 emergency declaration and widespread implementation of community mitigation measures, including school closures, social distancing, and mask wearing, although the exact timing varied by location (2). The decline in influenza virus circulation observed in the United States also occurred in other Northern Hemisphere countries (3,4) and the tropics (5,6), and the Southern Hemisphere temperate climates have had virtually no influenza circulation.
Although causality cannot be inferred from these ecological comparisons, the consistent trends over time and place are compelling and biologically plausible. Like SARS-CoV-2, influenza viruses are spread primarily by droplet transmission; the lower transmissibility of seasonal influenza virus (R0 = 1.28) compared with that of SARS-CoV-2 (R0 = 2–3.5) (7) likely contributed to a more substantial interruption in influenza transmission. These findings suggest that certain community mitigation measures might be useful adjuncts to influenza vaccination during influenza seasons, particularly for populations at highest risk for developing severe disease or complications.
Initially, declines in influenza virus activity were attributed to decreased testing, because persons with respiratory symptoms were often preferentially referred for SARS-CoV-2 assessment and testing. However, renewed efforts by public health officials and clinicians to test samples for influenza resulted in adequate numbers tested and detection of little to no influenza virus. Further, some countries, such as Australia, had less stringent criteria for testing respiratory specimens than in previous seasons and tested markedly more specimens for influenza but still detected few with positive results during months when Southern Hemisphere influenza epidemics typically peak. A new Food and Drug Administration–approved multiplex diagnostic assay for detection of both SARS-CoV-2 and influenza viruses could improve future surveillance efforts (https://www.cdc.gov/coronavirus/2019-ncov/lab/multiplex.html).
It is difficult to separate the effect that individual community mitigation measures might have had on influenza transmission this season. Although school-aged children can drive the spread of influenza, the effectiveness of school closures alone is not clear because adults have other exposures (8). There is evidence to support the use of face masks by infected persons to reduce transmission of viral respiratory illnesses to others and growing evidence to support their use (in the health care setting, in households, and in the community) to protect the healthy wearer from acquiring infection. More data are needed to assess effectiveness of different types of masks in different settings (9). Data from the current pandemic might help answer critical questions about the effect of community mitigation measures on transmission of influenza or other respiratory diseases. In addition, assessing acceptability of effective measures would be critical, because acceptability is likely to be inversely correlated with the stringency of the measure.
The findings in this report are subject to at least four limitations. First, an ecologic analysis cannot demonstrate causality, although the consistency of findings across multiple countries is compelling. Second, other factors, such as the sharp reductions in global travel or increased vaccine use, might have played a role in decreasing influenza spread; however, these were not assessed. Third, viral interference might help explain the lack of influenza during a pandemic caused by another respiratory virus that might outcompete influenza in the respiratory tract (10). This possibility is less likely in the United States because influenza activity was already decreasing before SARS-CoV-2 community transmission was widespread in most parts of the nation. Finally, it is possible that the declines observed in the United States were just the natural end to the influenza season. However, the change in the decrease percent positivity after March 1 was dramatic, suggesting other factors were at play.
The global decline in influenza virus circulation appears to be real and concurrent with the COVID-19 pandemic and its associated community mitigation measures.
Influenza virus circulation continues to be monitored to determine if the low activity levels persist after community mitigation measures are eased. If extensive community mitigation measures continue throughout the fall, influenza activity in the United States might remain low and the season might be blunted or delayed. In the future, some of these community mitigation measures could be implemented during influenza epidemics to reduce transmission, particularly in populations at highest risk for developing severe disease or complications.
However, in light of the novelty of the COVID-19 pandemic and the uncertainty of continued community mitigation measures, it is important to plan for seasonal influenza circulation this fall and winter. Influenza vaccination for all persons aged ≥6 months remains the best method for influenza prevention and is especially important this season when SARS-CoV-2 and influenza virus might cocirculate (1).
Regardless of how this year's flu season plays out - being part of a `vulnerable demographic' - I have every intention of getting the flu shot this month. I will also continue to social distance, practice stringent hand hygiene, and wear a face cover in public as part of a `layered' protection scheme.
I recognize the flu vaccine probably only provides my age group with 30%-40% protection, but given the long list of things that can go wrong during or following flu infection (see CDC: Another Study Linking Severe Influenza To Heart Damage), I'll take whatever advantage I can get.
Like wearing a seat belt in a car crash, getting the vaccine doesn't guarantee you'll walk away unscathed . . . but it does increase your chances.
