Ominous Flu Season Paired With COVID – What You Need To Know About Vaccination

A bad flu year on top of the pandemic could pose problems for hospitals that are already under pressure.

As winter looms and hospitals in the US continue to be flooded with severe cases of COVID-19, this year’s flu season poses a particularly ominous threat.

We are researchers with expertise in vaccination policy and mathematical modeling of infectious diseases. Our group, the Public Health Dynamics Laboratory at the University of Pittsburgh, has been modeling flu for more than a decade. One of us has served on the Centers for Disease Control and Prevention’s advisory committee on immunization practices and on the CDC’s Flu Vaccine Effectiveness Network.

Our recent modeling work suggests that last year’s slowed flu season could lead to an increase in flu cases next season.

Anti-COVID-19 strategies also reduced flu

As a result of the numerous measures taken in 2020 to curb the transmission of COVID-19 — including limiting travel, wearing masks, social distancing, closing schools and other strategies — the US saw a dramatic decline in flu and other infectious diseases during the past flu season.

Flu-related deaths in children fell from nearly 200 in the 2019-2020 season to one in the 2020-2021 season. Overall, the 2020-2021 flu season had one of the lowest recorded cases in recent US history.

While flu relief is a good thing, it could mean that the flu is hitting harder than usual this winter. This is because much of the natural immunity that people develop against disease comes from the spread of that disease through a population. Many other respiratory viruses showed a similar decline during the pandemic, and some of them, including interseasonal respiratory syncytial virus, or RSV, have increased dramatically as schools reopened and social distancing, masking and other measures have declined.

Deciphering Viral Transmission

Immunity to flu involves multiple factors. Flu is caused by different strains of an RNA virus that mutate at different rates each year, in a manner similar to the mutations that occur in SARS-CoV-2, the virus that causes COVID-19.

The level of a person’s existing immunity to the current year’s flu depends on several variables. They include how similar the current strain is to the one a child was first exposed to, whether circulating strains are similar to previously experienced strains, and how recent those influenza infections were, if they occurred.

And of course, human interactions, such as children crowding in classrooms or people attending large gatherings — as well as the use of protective measures like wearing a mask — all affect whether a virus is transmitted between humans.

There are also variables due to vaccination. The population’s immunity to vaccination depends on how many people get the flu vaccine in a given season and how effective — or well-tuned — that vaccine is against the circulating strains of flu.

The ongoing pandemic, coupled with the looming flu season, could lead to what doctors call “twin disease.”

There is no precedent for a ‘twindemic’

Given the limited spread of flu among the general U.S. population last year, our research suggests that the U.S. could experience a major flu epidemic this season. In combination with the existing threat of the highly contagious delta variant, this can lead to a dangerous combination of infectious diseases, or a ‘twindemic’.

Models of COVID-19 and other infectious diseases have been at the forefront of predictions about the COVID-19 pandemic and have often been shown to be predictive of cases, hospitalizations and deaths.

But there are no historical examples of this kind of double and simultaneous epidemic. As a result, traditional epidemiological and statistical methods are not well suited to predict what may happen this season. Therefore, models that explain the mechanisms of how a virus spreads are better able to make predictions.

We used two separate methods to predict the potential impact of last year’s drop in flu cases on the current 2021-2022 flu season.

In recent research of ours that has not yet been peer-reviewed, we applied a modeling system that simulates the interactions of a real population at home and at work, and at school and in the neighborhood. This model predicts that the US could see a major spike in flu cases this season.

In another preliminary study, we used a traditional infectious disease modeling tool that divides the population into those susceptible to infection, those infected, those who recovered, and those who were hospitalized or died. Based on our mathematical model, we predict that the US could see as many as 102,000 additional hospitalizations above the hundreds of thousands that normally occur during flu season. These figures assume that there will be no change in the usual intake and effectiveness of the flu vaccine from this fall and during the flu season.

Individual behavior and vaccination are important

A typical flu season usually causes 30 million to 40 million cases of symptomatic illness, between 400,000 and 800,000 hospitalizations, and from 20,000 to 50,000 deaths.

This prospect, combined with the ongoing battle against COVID-19, raises the possibility of a twentieth epidemic overwhelming the health care system as hospitals and ICUs in some parts of the country overflow with critically ill COVID-19 patients.

Our research also revealed how young children are particularly at risk, as they are less exposed to previous flu seasons and thus have not yet developed broad immunity compared to adults. In addition to the burden on children, childhood flu is a leading cause of flu in the elderly, as children pass it on to grandparents and other elderly people.

There is cause for optimism, however, as people’s behavior can significantly alter these outcomes.

For example, our simulation study included people of all ages and found that increasing childhood vaccination has the potential to cut children’s infections by half. And we found that if only 25% more people than usual get vaccinated against flu this year, that would be enough to bring the infection rate back to normal seasonal flu levels.

In the US, there is a lot of variation in vaccination coverage, adherence to social distancing recommendations, and mask wearing. So it’s likely that the flu season will experience significant variations from state to state, just as we’ve seen with patterns of COVID-19 infection.

All of this data suggests that while flu vaccination is important every year, this year it is paramount to prevent a dramatic rise in flu cases and to prevent US hospitals from becoming overburdened.

Written by:

Mark S Roberts – Distinguished Professor of Health Policy and Management, University of Pittsburgh Richard K Zimmerman – Professor of Family Medicine, University of Pittsburgh

This article was first published in The Conversation.