As winter tissues and hospitals across the United States continue to be exposed to severe cases of COVID-19, the flu season poses a particularly ominous threat this year.
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 the flu for over a decade.
One of us has been a member of the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices and the CDC’s Network Vaccine Effectiveness Network.
Our latest modeling work suggests that last year’s pumped-up flu season could lead to an increase in flu cases in the coming season.
Strategies against COVID-19 also reduced influenza
As a result of the many measures taken in 2020 to slow the transmission of COVID-19 — including travel restrictions, wearing masks, social distances, school closures, and other strategies — the United States experienced a dramatic decline in influenza and other infectious diseases. during the last flu season.
Influenza-related deaths in children dropped 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 numbers of cases in recent U.S. history.
Although flu reduction is a good thing, it can mean that flu will hit harder than usual this winter. This is because much of the natural immunity that humans develop against disease comes from the spread of this 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 have reopened and social distance, masking, and other measures have declined.
Decryption of viral transmission
Immunity to influenza involves several factors. Influenza is caused by several strains of an RNA virus that mutates at different rates each year in a way that is no different from 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 strain depends on several variables. They include how similar the current strain is to that a child was first exposed to, whether circulating strains resemble previously experienced strains, and how recent these flu infections were if they occurred.
And, of course, human interactions, such as children crowding into classrooms or people attending large gatherings – as well as the use of protective measures such as wearing a mask – all affect whether a virus is transmitted between people.
There are also variables due to vaccination. The population’s immunity to vaccination depends on the proportion of people who receive the flu vaccine in a given season and how effective – or well matched – that vaccine is against the circulating influenza strains.
There is no precedent for a ‘twindemic’
Given the limited spread of influenza in the general U.S. population last year, our research suggests that the United States could see a major flu epidemic this season. Paired with the existing threat of the highly infectious delta variant, this can result in 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 predictable for cases, hospitalizations and deaths.
But there are no historical examples of this type of dual and simultaneous epidemics. As a result, traditional epidemiological and statistical methods are not well placed to project what may happen this season. Therefore, models that incorporate the mechanisms of how a virus spreads are better able to predict.
We used two separate methods to predict the potential impact of last year’s fall in influenza cases on the current 2021-2022 influenza season.
In recent research of ours that has not yet been peer-reviewed, we have applied a modeling system that simulates the actual population interactions at home and at work and in school and neighborhood settings. This model predicts that the United States could see a large increase in influenza cases this season.
In another preliminary study, we used a traditional tool for modeling infectious diseases that divides the population into people susceptible to infection, the infected, the recovered, and those who have been hospitalized or died.
Based on our mathematical model, we predict that the United States could see up to 102,000 additional hospital admissions over the hundreds of thousands that typically occur during the flu season.
These figures assume that there is no change from the usual flu vaccine uptake and efficacy that starts in the fall and lasts through the flu season.
Individual behavior and vaccination are important
A typical flu season usually produces 30 million to 40 million cases of symptomatic disease, between 400,000 and 800,000 hospitalizations and from 20,000 to 50,000 deaths.
This prospect, coupled with the ongoing battle against COVID-19, increases the possibility of a twindemically overwhelming healthcare system as hospitals and ICUs in some parts of the country overflow with critically ill COVID-19 patients.
Our research also highlighted how young children may be particularly vulnerable as they have lower exposure to previous seasons of influenza and thus have not yet developed broad immunity compared to adults. In addition to the burden on children, childhood influenza is an important driver of influenza in the elderly, as children pass it on to grandparents and other elderly people.
However, there is reason for optimism, as people’s behavior can change these results significantly.
For example, our simulation study incorporated people of all ages and found that increasing vaccination among children has the potential to reduce infections in children by half. And we found that if only 25 percent more people than usual are vaccinated against the flu this year, that would be enough to reduce the rate of infection to normal seasonal flu levels.
Across the United States, there is great variation in vaccination rates, compliance with recommendations on social distancing, and mask use. So it is likely that the flu season will experience significant variation from state to state, just as we have seen with patterns of COVID-19 infection.
All of these data suggest that while influenza vaccination is important each year, it is of paramount importance this year to prevent a dramatic increase in influenza cases and prevent U.S. hospitals from being overwhelmed.
Mark S Roberts, Senior Professor of Health Policy and Management, University of Pittsburgh and Richard K Zimmerman, Professor of Family Medicine, University of Pittsburgh.
This article is republished from The Conversation under a Creative Commons license. Read the original article.