The COVID Column: Twindemic

Updated: Jun 21

by Finley Keene



If you recall, as the second COVID-19 surge began last year, scientists started worrying about what COVID and the annual flu combined might be like. Let’s take a look and see how we did this flu season in light of COVID-19.


The annual influenza pandemic has been around for many years, and the first time it struck hard was in 1918, killing off 50 million people. It was arguably one of the most deadly pandemics in history. It is still a major virus now, killing tens of thousands of people a year. This flu season, the infections and deaths from influenza have been dramatically low. What the scientists expected this year was a “twindemic”— two pandemics at the same time. The twindemic didn’t happen which was one piece of good news in an otherwise bleak year. However, because the flu numbers were so low this year, that might not be good news for subsequent years to follow.

The new numbers of flu cases for the 2020-21 season —which are not final yet— are far historically lower than previous years. Though the CDC has not released the statistics for the flu this season yet, KTVB, a newspaper local to Idaho, is reporting that there have only been 453 flu related deaths in the U.S. compared to 20,000 last year and 61,000 in the 2018-19 season. (If you are surprised by the amount of deaths for the 2018-19 season, the flu vaccine that year was not very effective against adults over 50, according to Pharmacy Times). As The Washington Post reports, there has been one pediatric (under 18) death, compared to 195 last year.

Not surprisingly, and directly related to the simultaneous COVID pandemic, the percentage of people who received the flu vaccine went way up. According to this CDC article, 193.8 million people in the U.S. got a flu shot this year, as compared to the 169 million vaccines given last year. This is a 13% increase, which is a big difference.

In addition, it’s possible that there was a lack of testing for the flu, since the scientific researchers were focussed on COVID-19 testing. This meant there were less cases recorded. Probably the biggest reason the flu was not pervasive this year is the mask wearing, social distancing, and hand washing. These are some things we’ve all known and practiced during this pandemic. As Rachel Baker, a public disease specialist at Princeton University says in this article, these COVID measures led to a 20 percent decrease in respiratory diseases.

In researching this article, I discovered that the CDC posts a Weekly Influenza Surveillance report. They collect data from 14 states, which represent 9 percent of the population. The report confirmed 223 influenza hospitalizations from October 1st, 2020 to April 30th, 2021. That means less than one hospitalization per 100,000 people. As the report noted, “This is lower than rates for any season since routine data collection began in 2005, including the low severity 2011-12 season. The current rate is one-tenth the rate during the 2011-12 season. Due to low case counts, only overall cumulative rates for the entire network are being reported this season.” This means that there hasn’t been a flu season like this, with these little cases and deaths, in a long time.

Just a couple days ago, I interviewed Dr. Tanya Madrid, an Internal Medicine Assistant Professor at Mt. Sinai, for some anecdotal as a primary care physician. She had some information on how the flu was this season as a person that experiences it from the front lines in terms of her patient care. When asked how her flu season was, she immediately said she had zero flu patients. Whatsmore, she only gave out about 5 flu tests in the entire season. She said that at the start of the season, she was very worried that the flu would strike hard, but then surprised and happy when there were no cases. She also said that all the practices, like wearing masks and social distancing, did in fact help.

In order to really see just how low the annual influenza pandemic was this year, let’s take a look at some data culled from the CDC on New York state’s positive flu tests and hospitalizations for this year compared to previous ones:


Just as a reminder, people likely tested for COVID instead of the flu, and that might be a reason such little cases were recorded. However, you can still conclude from these numbers that overall flu numbers were down this year.

As you can see, there are nearly no cases or hospitalizations in the 2020-21 season compared to last year and years before. This is another key indicator that this year’s flu pandemic in our state was dramatically lower. Now let’s take a look at some national numbers reported by the CDC so far:

“Influenza-like” means viruses that resemble influenza, so the flu isn't the only virus expressed in these numbers. Note how the 2019-2020 season was so high, due to COVID being new and possible conflation of the two by health care workers at the time.

So it’s great news that we did not have a twindemic this year, but what does a shortage of influenza cases mean for future flu vaccine development? The answer is unclear, but we may be in for some dangerous flu seasons ahead as a result.

The annual flu is the one that hit in 1918. In fact, it’s been around long before then, and the 1918 pandemic was just a major mutation in it. Now, how do we find the vaccine for the flu every year? According to this article by the CDC, the flu vaccine protects you against a couple different versions of the virus that seem like they might surge that year. They are found based on flu cases from science labs around the world.

For example, for the 2020-2021 flu shots, two of the officials chose to keep two of the three viruses in the old 2019-2020 flu shots, as there weren't a lot of chances for the virus to mutate. As we can see, a lot of how we make these vaccines depends on last year’s data. The main point of the article I mentioned is that there is a downside to no or low case count flu season. The downside is that we won’t have enough data for the flu vaccines next year.

Another big factor in the production of the vaccines is antigenic shift vs. antigenic drift. Basically, influenza is classified on two major proteins recognized by the immune system, the H and N proteins. Antigenic drift is a slight change in H or N protein. The body can still recognize it. Antigenic shift is where there is a H and N combination never seen before. Antigenic shift makes it hard to develop vaccines and bodies can't defend against the virus. The 1918 influenza pandemic represented an antigenic shift in the virus.

So if the annual influenza pandemic is still around from before 1918, does that mean COVID-19 could still be a pandemic 100 years from now? Like all other viruses, COVID mutates constantly. But whether it will pose a long term risk for humans is a question so far unanswered.

Recently, I interviewed Dr. Adam B. Keene (related), a Professor of Medicine at Montefiore Medical Center who specializes in Critical Care and Infectious Diseases. (A note to readers: as a budding journalist I am trying to expand my toolkit here and decided to incorporate a live interview into my column. But I’m also shy and didn't want to mess up my first interview so I decided to play it safe and ask an expert I know well, my dad). When asked whether we will need an annual new COVID shot, similar to the usual flu vaccine, Dr. Keene said the data is unclear. At the very least, we will probably have to get a booster shot at some point in our lifetimes.

On the topic of herd immunity, Dr. Keene said it would be very difficult to reach herd immunity if a large percent of the population doesn’t want the vaccine. Dr. Keene went on to say “COVID-19 does mutate quickly, but the way the flu mutates is different and important. Every year, we have influenza strains that evade the immunity created by our annual vaccines. Although it is theoretically possible, we have not yet seen COVID-19 strains that can evade the immunity created by the vaccines currently approved in the United States.”

The flu is and was a deadly virus, but the overwhelming majority of data from this year, along with opinions from experts, shows that it is becoming less of a concern. But even though we didn’t have to worry about it this year, this still can impact the vaccine for next year. Be ready for next year’s flu season!

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