Saturday, February 24, 2024

Frontiers Pub. Health: The Effect of Nonpharmaceutical Interventions (NPIs) on Influenza Virus Transmission

Photo Credit PHIL (Public Health Image Library)


Although their use was rare in western cultures, long before COVID burst onto the scene in late 2019 protective face masks were a common fashion accessory in Asia during flu season.  

So much so, that nearly a month before the first reports emerged out of Wuhan China, I wrote a `routine' blog about their use during Hong Kong's upcoming flu season (see HK CDW: Surgical Masks For Respiratory Protection).

Figure 2 - How to wear a surgical mask. (Source: The Centre for Health Protection of the Department of Health.

In that blog I wrote about the history of using face masks in Asia, one which goes back at least 100 years to the 1918 pandemic. Masks were used again after the great Tokyo earthquake and firestorm of 1923 to filter out the smoke and ash the lingered for months following the disaster.

Reinforced over the years by SARS, bird flu, three influenza pandemics, air pollution, and life in densely populated cities, the wearing of surgical masks in Asia has become commonplace.  

The use of NPIs (nonpharmaceutical interventions) like protective face masks, hand washing, and social distancing have long been a mainstay of pandemic planning (see 2017's Community Pandemic Mitigation's Primary Goal : Flattening The Curve), particularly during the opening months when no vaccine can be reasonably expected. 

While many people (understandably) don't like wearing them, all of the evidence to date shows that - when properly and consistently worn -  face masks (both surgical and N95/KN95) can greatly reduce your risk of infection from respiratory viruses. 

In 2022, an MMWR report found `Consistent use of a face mask or respirator in indoor public settings was associated with lower odds of a positive SARS-CoV-2 test result (adjusted odds ratio = 0.44). Use of respirators with higher filtration capacity was associated with the most protection, compared with no mask use.'

Four months ago, in JAMA Network: Masks During Pandemics Caused by Respiratory Pathogens—Evidence and Implications for Action, after presenting the available evidence, the authors wrote:

Available evidence strongly suggests that masking in the community can reduce the spread of SARS-CoV-2 and that masking with the highest-quality masks that can be made widely available should play an important role in controlling whatever pandemic caused by a respiratory pathogen awaits us.

Also last fall, we looked at a study which showed how quickly influenza and COVID rebounded in Hong Kong after their mask mandate was removed (see EID Journal: Influenza Resurgence after Relaxation of Public Health and Social Measures, Hong Kong, 2023).  

Today we've another research article, published in Frontiers of Public Health, which looks at the effectiveness of face masks in preventing influenza transmission in China; both before and during the COVID pandemic. 

One of the `downsides' to mask wearing is our natural exposure to respiratory viruses is limited, which can lead to less individual immunity over time, and a rebound in infections.  This is thought to have sparked the strong resurgence of influenza after mask mandates ended. 

While an expected side effect, its impact can probably be lessened by increased uptake of influenza (and/or pandemic) vaccines. 

Due to its length, I've only posted the abstract and some excerpts.  Follow the link to read the report in its entirety.  I'll have a postscript after the break. 

The effect of nonpharmaceutical interventions on influenza virus transmission

Danlei Chen1,2†Ting Zhang1†Simiao Chen2Xuanwen Ru2Qingyi Shao1,2Qing Ye2*Dongqing Cheng1*
Background: The use of nonpharmaceutical interventions (NPIs) during severe acute respiratory syndrome 2019 (COVID-19) outbreaks may influence the spread of influenza viruses. This study aimed to evaluate the impact of NPIs against SARS-CoV-2 on the epidemiological features of the influenza season in China.

Methods: We conducted a retrospective observational study analyzing influenza monitoring data obtained from the China National Influenza Center between 2011 and 2023. We compared the changes in influenza-positive patients in the pre-COVID-19 epidemic, during the COVID-19 epidemic, and post-COVID-19 epidemic phases to evaluate the effect of NPIs on influenza virus transmission.

Results: NPIs targeting COVID-19 significantly suppressed influenza activity in China from 2019 to 2022. In the seventh week after the implementation of the NPIs, the number of influenza-positive patients decreased by 97.46% in southern regions of China and 90.31% in northern regions of China.
However, the lifting of these policies in December 2022 led to an unprecedented surge in influenza-positive cases in autumn and winter from 2022 to 2023. The percentage of positive influenza cases increased by 206.41% (p < 0.001), with high positivity rates reported in both the northern and southern regions of China.

Conclusion: Our findings suggest that NPIs against SARS-CoV-2 are effective at controlling influenza epidemics but may compromise individuals’ immunity to the virus.


In the past 3 years, the widespread implementation of NPIs has greatly reduced the intensity of influenza virus infections. This may be beneficial in the short term, but research shows that immunization debt may have a greater negative impact (42). From 2019 to 2022, the activity of the influenza virus will be unprecedentedly suppressed. There is insufficient immune stimulation for people infected with influenza virus. When the susceptibility of the population increases, group immunity decreases, increasing the proportion of the population vulnerable to virus infection (43). As of April 25, 2023, the national influenza detection rate was 20.92% (48,527/231918), and the highest positive rate was 55.10%. The peak stage influenza positivity rate was higher than that prior to contracting COVID-19, and the highest level of influenza positivity has occurred since 2011. The 2023 influenza season is more severe than it was in previous years, with high influenza positivity rates that will lead to mass population infections in the near term. Schools were closed in many places, and hospitals saw a multifold increase in flu patients.

In conclusion, this study revealed that the low level of influenza activity in China from 2019 to 2021 was unprecedented, possibly due to the implementation of NPIs. This discovery has been confirmed in the United States and other studies. The high level of influenza activity in China in the fall and winter of 2022–2023 is likely a result of immune debt.
We summarized our experience with the COVID-19 outbreak. We found that we should spontaneously adopt nonpharmacological interventions, such as washing hands frequently, wearing masks and reducing people’s movement. Moreover, we can increase the influenza vaccination rate to minimize the negative impact of the outbreak. The weakness of this study is that the association between age and influenza was not analyzed. Future studies could focus on analyzing the disease burden of influenza in different age groups.

Face masks continue to be divisive, with many openly doubting their effectiveness.  

Last spring, many on social media were claiming a January Cochrane study proved that masks don't work.  But Karla Soares-Weiser, Editor-in-Chief of the Cochrane Library, issued a statement (see Cochrane Statement On Misinterpretations Of Their Mask Study) clarifying the study's findings.

Since COVID is unlikely to be the last severe pandemic threat we will face - or the deadliest - it is important that we better understand both the advantages and the limitations of wearing face masks.  

Masks aren't perfect; they can be uncomfortable or restrictive, good ones can be costly or in short supply, and they aren't 100% effective in preventing transmission. But when properly used they can be an effective protective measure until an effective vaccine can be distributed. 

Today, while supplies are abundant and prices are low, would be a good time to make sure you have an extra box or two of surgical masks, KN95s, or N95 respirators in your emergency kit. 

This was a recommendation I was making long before COVID emerged, and one I continue to make.