Until recently, wearing masks, a long-term custom in Japan during the winter flu season and during early springtime to protect against Japanese cedar pollinosis was not common in everyday life in other countries. Here, we provide our experience through the flu season of 2020/2021 in Japan, which underscores the importance of wearing masks to prevent coinfection of influenza and COVID-19.
COVID-19 emerged in 2019 and spread globally, challenging and threatening human lives in many countries. However, influenza virus infection is one of the common flu virus infections that is considered to be one of the leading causes of mortality in elderly individuals, particularly in those with pre-existing diseases, during the winter flu seasons worldwide.
Japan experienced strikingly minimal casualties during the influenza pandemic H1N1 in 2009 because of the strategy of vaccination combined with neuraminidase inhibitors to create herd immunity [1]. Since then, influenza flu shot with selective oral neuraminidase inhibitors are commonly used to prevent influenza in Japan, which attributed to the increased influenza vaccination in many countries [2].
Prior to the winter season of 2021, Japan faced serious concerns regarding co-infection of influenza and COVID-19, as both influenza infection and COVID-19 present with similar symptoms. Indeed, several cases of co-infection of COVID-19 with influenza were reported, which manifested with severe inflammatory reactions and organ injury [3].
Quite strikingly, however, based on the number of cases of influenza virus infection reported by the Ministry of Health, Labor and Welfare of Japan, and Infectious Disease Surveillance Center of Japan, the influenza pandemic level map showed over 1/2000 reduction (only 560 cases in the 5000 sentinel medical institutions (SMIs) in Japan) during the winter of 2021, despite influenza vaccination rates remaining at similar levels in recent years (Fig. 1 ).
Fig. 1.
Number of influenza infection cases and influenza vaccinations from 2012 through 2021
Number of influenza infection cases (line, cumulative number between January through March in each year from the 5000 sentinel medical institution (SMIs) in Japan) is superimposed on the number of influenza vaccinations in Japan (shaded bar).
Many other conditions and/or factors should be considered, such as possible cross inhibition by superinfection with other viral infections or inadequate protection against various respiratory viruses [4,5]. However, it appears that wearing masks, coupled with improvements in hygiene practices, likely contributed to the drastic decline in influenza cases, thereby minimizing co-infection.
COVID-19 emerged in 2019 and spread rapidly worldwide. Currently, we are still in the middle of developing efficient pharmacological therapies to combat COVID-19. Newly emerged variants, some of which are more contagious, are spreading globally, thereby posing a challenge to human life; however, currently, the available vaccines are promising in terms of prevention against the B.1.1.7 and B.1.351 variants [6]. The drastic decrease of influenza cases in 2020–2021 also revealed that the flu shot may need revaluation for its efficacy in flu prevention. It remains controversial whether wearing masks will be necessary after COVID-vaccination. Based on our experience, we strongly encourage people to wear masks in addition to getting vaccinated against flu for the successful prevention of co-infection of COVID-19 and influenza.
Declaration of Competing Interest
Both authors have no conflicts of interest to declare.
References
- 1.Okabe N. Epidemiological situation on pandemic influenza H1N1 2009 in the world and in Japan. JMA J. 2011;54(5):277–283. https://www.med.or.jp/english/journal/pdf/2011_05/277_283.pdf. [Google Scholar]
- 2.Reichert T.A., Sugaya N., Fedson D.S., Glezen W.P., Simonsen L., Tashiro M. The Japanese experience with vaccinating schoolchildren against influenza. N Engl J Med. 2001;344(12):889–896. doi: 10.1056/NEJM200103223441204. [DOI] [PubMed] [Google Scholar]
- 3.Cuadrado-Payán E., Montagud-Marrahi E., Torres-Elorza M., Bodro M., Blasco M., Poch E., et al. SARS-CoV-2 and influenza virus co-infection. Lancet. 2020;395(10236):e84. doi: 10.1016/S0140-6736(20)31052-7. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Rodgers L., Sheppard M., Smith A., Dietz S., Jayanthi P., Yuan Y., et al. Changes in seasonal respiratory Illnesses in the United States during the COVID-19 pandemic. Clin Infect Dis. 2021 doi: 10.1093/cid/ciab311. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.De Vries R.D., Schmitz K.S., Bovier F.T., Predella C., Khao J., Noack D., et al. Intranasal fusion inhibitory lipopeptide prevents direct-contact SARS-CoV-2 transmission in ferrets. Science. 2021;371(6536):1379–1382. doi: 10.1126/science.abf4896. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Abu-Raddad L.J., Chemaitelly H., Butt A.A. Effectiveness of the BNT162b2 Covid-19 vaccine against the B.1.1.7 and B.1.351 variants. N Engl J Med. 2021 doi: 10.1056/NEJMc2104974. [DOI] [PMC free article] [PubMed] [Google Scholar]