As flu season collides with the ongoing COVID-19 pandemic, people want to know what to expect with the holiday season nearing, air travel increasing and more workplaces requiring a full return to the office. Some things are clear: Demand for COVID-19 testing remains high, which has led Abbott to ramp up BinaxNOW production to more than 50 million tests per month at its U.S. manufacturing facilities, cases of the virus are on the rise in parts of the U.S. and the nation’s economy is showing signs of recovery. But much is still unknown. Cue Abbott scientists. Abbott has been at the forefront of the pandemic, helping develop 12 COVID-19 tests in 12 months. This accomplishment builds on Abbott’s 30 years of work tracking viruses around the globe and studying how they spread and mutate. Now, Abbott scientists are sharing some key insights into how the next few months might unfold and why a twindemic — a severe flu season during the COVID-19 pandemic — remains unpredictable. Dr. Norm Moore Director of Infectious Disease, Scientific Affairs at Abbott With flu season officially underway in the U.S., what did the season look like in the Southern Hemisphere and what does this mean for the U.S.? The Southern Hemisphere's flu season activity may predict what’s to come north of the equator, and the 2021 flu activity appears to have been lower than expected in the Southern Hemisphere. This is similar to what we saw in 2020 when flu was essentially nonexistent. Contributing to the low flu levels are likely the various hygiene and physical distancing measures people continue to follow to help prevent the spread of COVID-19. October is when you typically see flu activity increase in the Northern Hemisphere. It often peaks between December and February and winds down by May. As of the week ending Nov. 13, influenza activity remains low in the U.S. It will be important to stay vigilant as flu activity progresses throughout the remainder of this respiratory season. How will low population immunity to flu because of limited circulation of the virus last year affect this year's flu season? It takes about two weeks after getting the flu vaccine for antibodies to develop and provide protection against the influenza virus. And that protection typically lasts about six months, then protection declines, so the person’s antibody levels decrease. We also know that antibody responses caused by natural infection are often broader and longer lasting than an antibody response induced by vaccines. With so little flu circulating last year, it's likely that more people than usual are susceptible to the virus. Getting the flu vaccine could help decrease the severity of flu season, especially if the vaccine is a good match to the circulating viruses. Dr. Mary Rodgers, Ph.D. Principal Scientist for Abbott's Diagnostics business What about viral interference? Could COVID-19 keep flu transmission down or might an uptick in flu lead to a drop in COVID-19 cases? Viral interference is when infection with one virus provides protection against infection with other viruses. The thought is that a respiratory infection stimulates your immune response enough to keep other respiratory viruses from replicating in your cells and making you sick, at least for a period of time. Certainly you can get two viruses at the same time, like flu and COVID-19, but it's not common. It's possible that as COVID-19 continues spreading through communities this fall and winter (as is expected) it will contribute to once again lower than normal flu activity. This is why rapid COVID-19 testing remains a key tool, alongside COVID-19 and flu vaccines. Knowing whether or not you have COVID-19 or flu, in real time, enables you to minimize the spread of the viruses. It's likely that COVID-19 is here to stay and it’s possible that in the future, there will be separate peak seasons for COVID-19 and flu. How will the U.S. COVID-19 vaccination rate impact the ability of the virus to mutate and the types of mutations? All viruses mutate, some just change faster than others. And the more a virus circulates, the more opportunities it has to mutate. This is one reason why preventive measures — including testing, wearing a mask, washing your hands and getting vaccinated — are important. We know that when a high percentage of a population has immunity to a virus, the spread of the virus is greatly reduced. This is known as herd immunity. The amount of immunity needed to reach this threshold depends on the contagiousness of a virus. For instance, to achieve herd immunity against measles, about 95% of a population has to be immune to the virus, while the threshold for polio is about 80%. Scientists are still learning about immunity to COVID-19, including how strong it is, how long it lasts and how much herd immunity is needed for the virus. Learn more about Abbott’s continued work to fight the still evolving threat of COVID-19. This story was originally published on November 24, 2021 and updated on June 03, 2022.