#17,738
As I've done every year for more than 2 decades, yesterday I got my annual flu shot. As I stood in a short queue of people at the pharmacy, one lady picking up a prescription was coughing noticeably.
But since I'd recently had my updated XBB COVID shot - and was wearing a KN-95 face mask - I wasn't terribly concerned.
Face masks have been de rigueur in Japan, and much of Asia, for decades during respiratory season (see 2019 pre-pandemic HK CDW: Surgical Masks For Respiratory Protection), but their use elsewhere really only took off with the arrival of COVID in the spring of 2020.
Unfortunately anti-mask sentiment today is almost as strong as anti-vaccine rhetoric. For many, I suspect mask wearing is too much of a reminder of the first two miserable years of the pandemic. So much so that many simply refuse to wear them.
Of course, everyone has to make their own risk-reward calculation. What might be a devastating (even fatal) respiratory infection for me at the age of 70 (see PloS One: Early Risk of Acute Myocardial Infarction Following Hospitalization for Severe Influenza), might be viewed as far less risky by someone much younger.While I certainly don't enjoy wearing them (especially during our oppressive summer months), I've never stopped wearing them in indoor, crowded, environments. And given the benefits - and my age - I imagine I'll continue to do so for some time to come.
There remains some heated debate over just how effective masking really is in reducing respiratory infections. Much, of course, depends on the type of mask, and whether it is worn properly and consistently.
Some people insist on citing a misinterpreted Cochrane study (see Cochrane Statement On Misinterpretations Of Their Mask Study) as `proof' they don't work, while ignoring other studies (see
MMWR: Relative Effectiveness Of Different Mask Types In Preventing COVID Infection) showing they that they are effective.
MMWR: Relative Effectiveness Of Different Mask Types In Preventing COVID Infection) showing they that they are effective.
Today we've a new study, this time from Hong Kong, on the effectiveness of face masks in preventing influenza infection.
During the first 3 years of the COVID pandemic, Hong Kong had one of the most restrictive mask mandates in the world, and during that time influenza was practically non-existent. But within days of removing that mandate (March 1, 2023), Hong Kong saw an explosive rise in influenza infections.
Considering the simultaneous abandonment of other public health and social measures (PHSMs), parsing out how much of an impact face masks had in preventing influenza isn't an easy task, and a precise calculation is probably impossible.
But after considerable statistical gymnastics (see details below), the authors estimated that face masks provided a 25% reduction in influenza transmission.
When you add that layer of protection to getting the yearly flu vaccine, rigorous hand hygiene, and avoiding crowds whenever possible, you should be able to reduce your risk of infection significantly.
Volume 29, Number 12—December 2023
Research Letter
Influenza Resurgence after Relaxation of Public Health and Social Measures, Hong Kong, 2023
Weijia Xiong, Benjamin J. Cowling, and Tim K. Tsang
Author affiliations: The University of Hong Kong School of Public Health, Pokfulam, Hong Kong, China (W. Xiong, B.J. Cowling, T.K. Tsang); Hong Kong Science and Technology Park, Pak Shek Kok, Hong Kong (B.J. Cowling, T.K. Tsang)
Abstract
Soon after a mask mandate was relaxed (March 1, 2023), the first post–COVID-19 influenza season in Hong Kong lasted 12 weeks. After other preventive measures were accounted for, mask wearing was associated with an estimated 25% reduction in influenza transmission. Influenza resurgence probably resulted from relaxation of mask mandates and other measures.
To control COVID-19, Hong Kong, China, put in place several public health and social measures (PHSMs), including mandatory mask wearing, school closures, hand hygiene, and avoidance of gatherings. In early 2020, those measures also reduced influenza transmission (1), and according to laboratory surveillance records, influenza virus did not circulate in the community for 3 years (2). From mid-2022 through 2023, PHSMs were progressively relaxed, and on March 1, 2023, the local mask mandate was lifted. We investigated the effects of PHSMs on influenza transmission in Hong Kong.
We collected weekly influenza-like illness consultation rates reported by private general practitioners and the weekly proportion of sentinel respiratory specimens that tested positive for influenza virus in Hong Kong during October 2010–May 2023. We established a proxy for influenza virus activity by multiplying rates of influenza-like illness by the proportion of influenza-positive samples following previous studies (3,4) (Appendix). We found that weekly influenza activity had decreased to almost zero since March 2020, when PHSMs against COVID-19 began (Figure). Before mandatory on-arrival quarantine of travelers started on September 26, 2022, only sporadic influenza-positive samples were detected by surveillance, all from travelers or children who had recently received live-attenuated influenza vaccine (5). After travel restrictions were removed, sporadic influenza detections increased, but overall activity remained low. After mandatory indoor and outdoor mask wearing restrictions were lifted on March 1, 2023, influenza transmission increased substantially; the first influenza season after COVID-19 in Hong Kong started and peaked on April 9, ended on May 25, and lasted for 12 weeks (6).
Because various other PHSMs were implemented concurrently with the mask mandate, resurgence of influenza activity could not be attributed to relaxation of the mask mandate alone. Therefore, we used a previous approach that estimated the time-varying effective reproductive number (Rt) (7) and a multivariable log-linear regression model on Rt that could allow for adjustment of other factors affecting influenza transmission, including depletion of susceptible persons, seasonal differences, and meteorologic predictors and preventive measures (Appendix). Because the predominating influenza strain in 2023 was influenza A(H1N1)pdm09, we identified previous influenza A(H1N1)pdm09 epidemics that had occurred during 2010–2020. To construct a preventive score, we used data from cross-sectional telephone surveys among the general adult population in Hong Kong from 2020 to 2023 as a proxy for the intensity of preventive measures, other than mask wearing, against COVID-19 (1). The preventive score included the average proportion of persons who avoided visiting crowded places, avoided going to healthcare facilities, avoided touching public objects, or used protective measures when touching public objects, and washed hands immediately after going out. Before 2020, the proportion of those preventive measures was established as baseline. When constructing a preventive score, we compared the Akaike information criterion of 4 combinations of those protective measures. Meteorologic variables provided by the Hong Kong Observatory (http://hko.gov.hkExternal Link) were temperature, wind speed, and relative and absolute humidity. To quantify the effects of meteorologic variables, we fitted the models to data before the COVID-19 pandemic.
Among the 9 epidemics of 2010–2023, the estimated Rt varied from 0.62 to 1.38 (median 1.02) (Appendix Figure 1). The estimated Rt showed a decreasing pattern in each season, ranging from ≈1.2 at the beginning of an epidemic period to 0.8 at the end of an epidemic period. After model selection (Appendix), we found that a model of absolute humidity, mask wearing, and preventive score 3 (Table) explained 92% of the observed variance in estimated Rt (Appendix Table 1). Changes in absolute humidity (Appendix Figure 2, panel A), the proportion of mask wearing, and preventive score 3 (Appendix Figure 2, panel B) strongly correlated with changes in Rt. After adjusting for other factors, such as depletion of susceptible persons, between-season effects, and absolute humidity, we found that mask wearing was associated with a 25% (range 1%–43%) reduction in Rt and that other preventive measures (combined) were associated with a 77% (range 60%–88%) reduction (Table).
We found that that influenza increased after PHSMs were relaxed and influenza transmission increased shortly after the mask mandate was relaxed. Our results are consistent with those of several studies that found that PHSMs against COVID-19 may reduce influenza transmission (8) and that mask wearing may have a low to moderate protective effect against influenza virus transmission in the community (9,10).
A limitation of our analysis was that we used results of survey reports to generate a proxy of intensity of implemented PHSMs over time, which may not be accurate. Also, we used a proxy measure of influenza activity based on surveillance data, and the reliability of our analysis depended on the accuracy of this proxy. In addition, influenza vaccination coverage (Appendix Figure 5) was not included in the model because our model included the effect of vaccination via season-specific intercept. Nevertheless, our study results suggest that the resurgence of influenza after relaxation of PHSMs was most likely affected by the lifting of mask mandate and other PHSMs.
Ms. Xiong is a PhD candidate at the School of Public Health, University of Hong Kong. Her research interests are infectious disease epidemiology and modeling and development of statistical approaches for infectious disease analysis.
