We fast-track accepted articles to ensure that important scientific research is shared as rapidly as possible and publish them as edited preprints, prior to publication in the IJTLD or PHA.
C. C. Leung, T. H. Lam
Over the past three years, COVID-19 control measures have led to an unintentional lowering of the prevalence of common respiratory infections.1,2 Herd immunity has accumulated as a result of natural virus circulation and vaccination, so non-pharmaceutical COVID-19 control measures have been progressively withdrawn.3 In the United States, there are alarming signs of a tridemic involving COVID-19, influenza, and respiratory syncytial virus (RSV).4–6
Advances in research on COVID-19 and aerosol dynamics reveal the key role of airborne transmission for SARS-CoV-2 and other common respiratory viruses.7,8 However, current infection control guidelines have not been updated to highlight this newly acquired knowledge.9 The well-known hierarchy of administrative control, environmental control and personal protection was established largely through earlier experiences with TB control.10 These measures need to be urgently updated to overcome the challenges from faster virus shedding, shorter incubation periods and greater transmissibility of many respiratory virus infections. Administrative control is much more difficult with a virus that can spread rapidly before symptoms occur or in asymptomatically infected individuals,11–13 particularly if a highly sensitive point-of- care diagnostic test is not available. Ventilation remains useful for long-range aerosol transmission, but ventilation dilution is too slow to stop transmission at short range or when high infective loads are continuously emitted by breathing, talking, singing, coughing or sneezing, particularly within an elated gathering of many individuals in confined settings.7,8
Wearing a well-fitting mask can reduce transmission, both by controlling the source and by providing personal protection.11,14 If both parties wear a mask that is 80% effective for source control and personal protection, their combined effect would reduce the risk to (1 - 0.8) x (1 - 0.8) = 4%, compared to when only one party wears a mask the risk is 1 x (1 - 0.8) = 20%. Existing advice to ask only symptomatic or vulnerable persons to wear mask is less effective than universal masking. Transmission often occurs in asymptomatic or pre-symptomatic persons for COVID-19, influenza or RSV.11–13 In the Greater Boston area of Massachusetts, United States, the lifting of universal masking requirements in school districts led to a doubling of case rates among both students and staff in the following 15 weeks compared to school districts that sustained the mandatory requirement, even when their baseline rates were similar.15 This could have underestimated the actual protective effect, as students and staff in all school districts were similarly exposed to other infectious sources outside schools following the general relaxation of the masking requirement in Massachusetts. As shown in the Figure, the theoretical protective effectiveness of masks increases rapidly with increasing mask coverage. Using an 80% effective mask (for source control and recipient protection), the overall transmission risk increases from 4% (of the unmasked level) when everyone in a group wears a mask, to 36% when only half do so.
The waning or low herd immunity for COVID-19, influenza and RSV could interact with behavioural changes and colder weather to trigger a tridemic of these airborne respiratory infections in the northern hemisphere this winter.4,16Vaccination access and/or uptake may still remain suboptimal for COVID-1916,17 and influenza18 in many countries, and vaccines for RSV are still in development. Although oral antivirals are now available for COVID-19 and influenza, they may not be easily accessible or affordable for relevant high-risk groups, especially in low- to middle-income countries. Masking is a readily available and highly effective tool to control airborne transmission of these common respiratory infections. Unfortunately, strong public desire to return to normality may have generated an overt or subtle social pressure to remove the mask,16 although this simple tool can potentially save lives in aggregate settings.19,20 Social prejudice and political hurdles may also hinder the re-introduction of masking requirements, at least indoors. Cultural norms differ between countries and ideological differences also affect how one perceives the role of the mask. However much we desire to resume a “normal” life, during a tridemic, altruism and solidarity are needed to promote masking to protect large, vulnerable segments of our population.21 A debate on this issue by the medical and scientific communities is most welcome.
- Wong NS, Leung CC, Lee SS. Abrupt subsidence of seasonal influenza after COVID-19 outbreak, Hong Kong, China. Emerg Infect Dis 2020;26:2753–2755.
- Sullivan SG, et al. Where has all the influenza gone? The impact of COVID-19 on the circulation of influenza and other respiratory viruses, Australia, March to September 2020. Euro Surveill 2020;25:2001847.
- Migliori GB, et al. Country-specific lockdown measures in response to the COVID-19 pandemic and its impact on tuberculosis control: a global study. J Bras Pneumol 2022;48:e20220087.
- Tanne JH. US faces triple epidemic of flu, RSV, and covid. BMJ 2022;379:o2681.
- Centers for Disease Control and Prevention. Weekly U.S. Influenza Surveillance Report. Atlanta, GA, USA: CDC, 2022.
- Centers for Disease Control and Prevention. The National Respiratory and Enteric Virus Surveillance System. RSV National Trends. Atlanta, GA, USA: CDC, 2022.
- Wang CC, et al. Airborne transmission of respiratory viruses. Science 2021;373:eabd9149.
- Leung NHL. Transmissibility and transmission of respiratory viruses. Nat Rev Microbiol 2021;19:528–545.
- Klompas M, et al. Current insights into respiratory virus transmission and potential implications for infection controlprograms: a narrative review. Ann Intern Med;174:1710–1718.
- Centers for Disease Control and Prevention. Guidelines for preventing the transmission of Mycobacterium tuberculosis in health-care facilities. MMWR 1994;43(No. RR-13):1–132.
- Esposito S, et al. Universal use of face masks for success against COVID-19: evidence and implications for prevention policies. Eur Respir J 2020;55:2001260.
- Cohen C, et al. Asymptomatic transmission and high community burden of seasonal influenza in an urban and a rural community in South Africa, 2017-18 (PHIRST): a population cohort study. Lancet Glob Health 2021;9:e863–e874.
- Otomaru H, et al. Risk of transmission and viral shedding from the time of infection for respiratory syncytial virus in households. Am J Epidemiol 2021;190:2536–2543.
- Leung CC, Lam TH, Cheng KK. Mass masking in the COVID-19 epidemic: people need guidance. Lancet 2020;395:9455.
- Cowger TL, et al. Lifting Universal Masking in Schools - Covid-19 Incidence among Students and Staff. N Engl J Med. 2022 Nov 9. doi: 10.1056/NEJMoa2211029. [Online ahead of print.]
- Leung CC, Lee ACK. Are we coming out from the COVID-19 pandemic? Respirology 2022 Nov 1; doi: 10.1111/resp.14403. [Online ahead of print.]
- Andreas M, et al. Interventions to increase COVID-19 vaccine uptake: a scoping review. Cochrane Database Syst Rev 2022;8: CD015270.
- Centers for Disease Control and Prevention. Weekly flu vaccination dashboard. Atlanta, GA, USA: CDC, 2022.
- Leung CC, et al. Mask wearing to complement social distancing and save lives during COVID-19. Int J Tuberc Lung Dis 2020;24:556–558.
- Wildenbeest JG, et al. The burden of respiratory syncytial virus in healthy term-born infants in Europe: a prospective birth cohort study. Lancet Respir Med 2022:S2213-2600(22)00414-3. [Online ahead of print].
- Cheng KK, Lam TH, Leung CC. Wearing face masks in the community during the COVID-19 pandemic: altruism and solidarity. Lancet 2022;399:e39–e40.