With winter approaching and hospitals across the United States continuing to be overwhelmed with serious cases of COVID-19, flu season presents a particularly ominous threat this year.
Researchers with expertise in vaccination policy and mathematical modeling of infectious diseases at the University of Pittsburgh, including Mark S. Roberts and Richard K. Zimmerman, professor of family medicine, have been working on influenza modeling for more than a decade.
One of them was a member of the Centers for Disease Control and Prevention’s Immunization Practices Advisory Committee and the Centers for Disease Control and Prevention’s Influenza Vaccine Effectiveness Network.
Recent modeling work suggests that last year’s mild flu season could lead to an increase in flu cases next season. Strategies to combat “Covid-19” have also reduced influenza.
As a result of the many measures put in place in 2020 to reduce the transmission of COVID-19—including limiting travel, mask wearing, social distancing, school closures, and other strategies—the United States experienced significant reductions in influenza and other infectious diseases during the past influenza season.
Influenza-related deaths among children fell from around 200 in the 2019-20 season to one in the 2020-2021 season. Overall, the 2020-2021 flu season saw one of the lowest recorded numbers of cases in recent U.S. history.
And while curbing the flu is a good thing, it could mean that the flu will hit more than usual this winter. This is because much of the natural immunity that people develop to the disease comes from the spread of the disease in the population.
Many other respiratory viruses showed similar declines during the pandemic, and some, including inter-class respiratory virus, increased dramatically as schools reopened and social distancing, concealment and other measures decreased.
Decoding the viral transmission
Immunity to influenza involves several factors. Influenza is caused by several strains of the RNA virus that mutate at different rates each year, not unlike the mutations that occur in SARS-CoV-2.
The current level of immunity of an individual against the influenza strain for the current year depends on several variables. They include how similar the current strain is to the strain the child was first exposed to, whether the circulating strains are similar to strains they have been exposed to, and how recent this influenza infection is, if it occurs.
Of course, human interactions, such as children gathering together in classrooms or people attending large gatherings – as well as the use of preventive measures such as wearing a mask – all affect whether the virus is transmitted between people.
There are also variants due to vaccination. A population’s immunity to vaccination depends on the proportion of people who receive the flu vaccine in a particular season and how effective – or well-matched – the vaccine is against circulating flu strains.
Given the limited prevalence of influenza in the general US population last year, the research suggests that the US could experience a major influenza pandemic this season. Combined with the current threat of the highly contagious delta variant, this could lead to a dangerous mix of infectious diseases, or ‘twins’.
Models of COVID-19 and other infectious diseases have been at the forefront of predictions about the COVID-19 pandemic, and have often proven to be predictive of cases, hospitalizations, and death.
However, there are no historical examples of this kind of dual and simultaneous epidemics. As a result, traditional epidemiological and statistical methods are not well suited to predicting what may happen this season. Therefore, models that integrate the mechanisms of how the virus spreads are better able to make predictions.
The researchers used two separate methods to predict the likely impact of last year’s decrease in influenza cases on the current 2021-2022 influenza season.
In the latest research, which has not yet been peer-reviewed, we applied a modeling system that simulates actual residents’ interactions at home, at work, at school, and in the neighborhood. This model predicts that the United States may see a significant rise in influenza cases this season.
In another preliminary study, we used a traditional infectious disease modeling tool that divides populations into susceptible, infected, recovered, and hospitalized or dead people.
Based on the mathematical model, we predict that the United States could see as many as 102,000 additional hospitalizations above the hundreds of thousands that typically occur during flu season.
These numbers assume that there is no change from the uptake and effectiveness of the usual flu vaccine starting in the fall and continuing through the flu season.
A typical influenza season results in 30 to 40 million cases of symptomatic illness, 400,000 to 800,000 hospitalizations, and 20,000 to 50,000 deaths.
This possibility, combined with the ongoing battle against COVID-19, raises the prospect of a massive pandemic on the healthcare system as hospitals and intensive care units in some parts of the country are overflowing with critically ill COVID-19 patients.
The research also sheds light on how young children are particularly vulnerable because they are exposed to less pre-flu seasons and therefore 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, because people’s behaviors can greatly alter these findings.
Source: Science Alert