By Dr. Peter Palese, Ph.D.

“Twindemic” is a term that has not yet been in the headlines but is likely to become part of our lexicon as the summer progresses. Twindemic refers to the possibility that we will see continuing cases of COVID-19 together with severe influenza forcing the country to deal with two serious diseases at the same time.

We are all familiar with the annual flu vaccines that have helped to control and limit influenza cases.  Influenza has been with humans for centuries, and certainly since the 1918/1919 “Spanish flu” pandemic we have respect for a disease that  caused 50-100 million deaths worldwide just 100 years ago. There were additional influenza pandemics in 1957, 1968, and in 2009, with estimated numbers of worldwide deaths of 1.1 million, 1 million, and 284,000, respectively. In addition to pandemics, seasonal influenza has taken its toll as well. In the U.S., the 2018-2019 influenza season counted 35 million cases, 490,000 hospitalizations, and 34,000 deaths.

With the onset of the COVID-19 outbreak (declared a pandemic by the WHO in March 2020), we have all experienced an unforeseen and unpredicted catastrophe resulting in almost 4 million deaths worldwide with more likely. Hopefully the availability of a sufficient number of vaccine doses and the timely distribution globally will bring the pandemic under control. Unfortunately, the emergence of SARS-CoV-2 variants may complicate our efforts to achieve this goal. Time will tell if additional variant strains will emerge which make our present vaccines unable to efficiently protect against COVD-19 infection. In addition, we still do not know whether SARS-CoV-2 will change over time leading to a situation  similar to that observed with influenza viruses,  which has created the need  for an annual revaccination with a formulation of strains matching the actual variants circulating in the population. In any case, we have entered an unprecedented period of two pandemic pathogens — influenza viruses and SARS-CoV-2 –, threatening our fabric of life all over the planet.

Several important and troubling questions arise as to the possible impact  of these two simultaneous infections:

1) Did influenza cases abruptly decline last winter  as people stayed isolated and wore masks because of widespread COVID-19 outbreaks?

The effects of social distancing and of wearing masks on the transmission of influenza viruses had previously never been seriously studied. Thus, it was/is not known what impact a nationwide social distancing and mask wearing campaign brought on by the COVID-19 outbreak would do to the human-to-human transmission of other respiratory illnesses. However, the simple explanation of maintaining appropriate distances between people and cutting down on aerosol transmission by nose and mouth protection appears to be accepted by many  to be responsible for dramatically lower influenza rates (increased hand washing also may have played a part). In fact, the incidence of influenza including the death rate (winter 2020/2021) was one hundred times lower than in most other years in the U.S.

2) Should we anticipate that influenza will return this  fall/winter?

If social distancing and mask wearing  led to drastically lower influenza cases since last winter, we should be prepared for an increase in influenza virus infections for the 2021/2022 winter because of the lifting of the COVID-19 restrictions. We already see this year an increase in respiratory syncytial virus, regular coronavirus, and rhinovirus infections which may all be related to the relaxation of anti-COVID-19 measures. This scenario may forewarn of a more serious winter season in terms of the increased number of influenza infections and of other seasonal pathogens than what was observed during the 2020-2021 season.

3)  Should we plan to be vaccinated against both viruses?

Although influenza viruses and SARS-CoV-2 both  can cause respiratory diseases, they are very different pathogens. Vaccines made for one virus are not effective in curbing replication of the other. Vaccination with one vaccine does not interfere with protection afforded by the other vaccine. Thus, vaccination against both viruses is highly recommended.

4) Do people need to be vaccinated against both pathogens?

Fortunately, we have vaccines which are protective against the COVID-19 virus and  other vaccines which are effective against influenza viruses. In both cases, vaccination can prevent serious disease and complications caused by natural infections. Despite fears by some people, the benefits of vaccination against these vaccine-preventable diseases are extraordinary and should be foremost in our minds to help protect our health. One of the intractable public health issues remains the strength of anti-vaxxers who believe that vaccinations are harmful to human health among many arguments against vaccination. Most harmful, they refuse to vaccinate their children and perpetuate their anti-vaxxer position.

5) Do masks protect against influenza virus infections?

The wearing of masks also prompts social distancing so that it may be difficult to separate the benefits of keeping greater distances as compared to preventing aerosol transmission through face covering. We will have to intensify studies addressing this question and to find solutions to better protect us against respiratory agents in general. Such non-medicinal approaches may be important future strategies to curb respiratory diseases including influenza.

6) Will we need booster vaccinations against SARS-CoV-2?

In the U.S., several COVID-19 vaccines have received Emergency Use Authorization by the FDA and the Modern and Pfizer have both submitted data to the FDA for full approval of their vaccines. These vaccines have been found to be extremely safe and effective. In fact, they have shown unbelievable protection rates against the Wuhan strain of the virus as well as variants identified in the UK, South Africa, Brazil, and India. Since the virus has been circulating for roughly 18 months worldwide and vaccines have been available for less than a year, the question of duration of protection remains unknown.  It is safe to suggest that vaccinees will be properly protected at least until the end of this calendar year (2021). Clinical data of vaccinated and naturally infected patients, yet to be obtained, will give us answers to the question of booster vaccinations in the future years. Fortunately, our technical and medical advances will provide us with effective tools to tame these threating pathogens.

We must recognize how fortunate we are.  Even the simultaneous presence of two pandemic agents – the so-called “twindemic” — does not represent the threat it could be because we have effective vaccines against both, as well as  potent antiviral drugs to treat influenza viruses.

So, people who want to protect themselves should plan on getting the influenza vaccine early this fall and await guidance, based on emerging science, on whether and when a booster is needed for SARS-CoV-2.  To the extent appropriate, particularly when it comes to indoors versus outdoors, continuing social distancing and mask wearing affords still greater protection from both viruses.

Over the past 18 months we’ve come a long way in understanding viruses and in implementing a remarkable response. But we are not yet finished with the pandemic, or perhaps better put the pandemic is not yet finished with us. But I am confident, like many other great health challenges we have faced previously, we will overcome this!


Peter Palese, Ph.D. is Professor of Microbiology and Chair of the Department of Microbiology at the Icahn School of Medicine at Mount Sinai; Center of Excellence Director, Global Virus Network (GVN), headquartered in Baltimore, Maryland; and is a member of the National Academy of Sciences and the Institute of Medicine. He is a recognized expert in RNA viruses.