Note: The numbers reflect the weekly totals of positive flu tests from public and clinical laboratories.· ·Source: Centers for Disease Control and Prevention
There have been fewer cases of influenza in the US this flu season than ever before. According to the Centers for Disease Control and Prevention, about 2,000 cases have been recorded since the end of September. Over the past few years, the average number of cases over the same period has been around 206,000.
When measures were taken across the country to contain the spread of the coronavirus in March 2020, the influenza quickly disappeared and has still not returned. The last flu season, which normally would have lasted until next month, essentially never happened.
After fears that a “twinkle chemistry” could hit the country, the absence of the flu was a much-needed respite that eased the strain on an overwhelmed health system. However, the lack of exposure to the flu could also leave the population more vulnerable to the virus if it returns – and experts say its return is safe.
"We don't know when it will come back in the US, but we know it will come back," said Sonja Olsen, epidemiologist at the C.D.C.
Experts are less sure what will happen when the flu returns. In the coming months – when millions of people return to public transport, restaurants, schools, and offices – influenza outbreaks could be more widespread than normal or occur at unusual times of the year. However, it is also possible that the returning virus may be less dangerous as it did not have a chance to develop during the break.
"We have no real clue," said Richard Webby, a virologist at St. Jude Children's Research Hospital in Memphis. "We're in unknown territory. We haven't had such a low influenza season, I think, as long as we've measured it. So the possible effects are a bit unclear."
Scientists don't yet know which public health actions were most effective at helping eradicate the flu this season. However, if behaviors such as wearing masks and frequent hand washing continue after the coronavirus pandemic ends, they could help keep influenza at bay in the United States.
Much also depends on the latest flu vaccines, their effectiveness, and the willingness of the public to receive them. However, the recent decline in cases has made it difficult for scientists to decide which strains of flu these vaccines should protect against. It is more difficult to predict which strains will circulate later when so few are circulating now.
What happened to the flu?
When the reality of the coronavirus pandemic hit last year, the country was still in normal flu season, which peaked in February. Then schools closed, travel stopped, millions started working from home, and the number of new flu cases quickly plummeted to all-time lows even as the coronavirus spiked.
Influenza vs. Coronavirus
Flu cases even fell as the coronavirus spread.
Note: The numbers represent the weekly totals.· ·Sources: Centers for Disease Control and Prevention (Influenza Cases); New York Times database of state and local health reports (coronavirus cases)
And the decline isn't due to a lack of testing. 1.3 million specimens have been tested for influenza since the end of September, more than the average of about one million over the same period in recent years.
Public exposure to influenza in public could, according to scientists, partly explain why the flu practically disappeared while the coronavirus continued to spread after security measures were implemented.
"For something like Covid, which has a completely vulnerable population at the beginning of a pandemic, a lot more work is needed to slow the spread of the infection," said Rachel Baker, an epidemiologist at Princeton University.
In other words, unlike the coronavirus, the population has natural immunity to the flu, having been exposed to different strains of the virus for years. People are susceptible to new strains of flu every year, but less than to completely unknown viruses.
The very presence of the coronavirus could also have played a role in suppressing cases of flu, said Dr. Webby, as there is often only one dominant respiratory virus in a population at any given time. "One tends to keep the other out," he said.
And influenza wasn't the only virus to go away in the last year. There have also been significant decreases in other respiratory diseases, including Respiratory Syncytial Virus, or R.S.V., which is the leading cause of pneumonia in infants.
What if the flu returns?
Influenza is a relatively common illness that can be fatal, especially in young children, the elderly, and adults with chronic illnesses. The C.D.C. It is estimated that the flu has killed 12,000 to 61,000 people annually since 2010.
If immunity to the flu declines during the pandemic due to lack of exposure to the latest strains of flu, more people than usual may be susceptible to the virus.
"Every year, exposure to the flu virus boosts and stimulates immunity of 20 to 30 percent of the population," said Dr. Webby. "We won't have that this year."
"A decrease in natural immunity is a problem," said Dr. Olsen, "and lower immunity could lead to more infections and more serious illnesses."
The result could be major outbreaks of the flu and the R.S.V. mean out of season, said Dr. Baker. In Florida, R.S.V. would normally be bearish at this time of year but there is an upward trend right now.
When larger numbers of offices and schools reopen in the fall, as many expect, scientists will look closely.
"We are always concerned about influenza, which causes serious illness, especially in those at increased risk of complications," said Dr. Olsen. “We know that school-age children are important drivers of influenza virus transmission. However, because influenza is difficult to predict, we cannot predict the severity of the next season. "
The absence of influenza also has a potential benefit: fewer cases usually lead to fewer mutations.
"At the moment, it is possible that because the influenza is not circulating as widely, the virus did not have as many opportunities to develop," said Dr. Baker, "which means our vaccines might be more effective than normal."
Choosing the strains for the flu vaccine
Developing the influenza vaccine this year has been more difficult than it has been in the past.
Each year, scientists assess the strains of influenza circulating around the world and meet to decide which strains this year's vaccine should protect against. They study the strains that make people sick and use this information to predict which strains are most likely to infect people when the flu season hits.
"We met in late February to make these recommendations," said Dr. Webby, referring to the World Health Organization panel that evaluates the flu vaccine. “And it was difficult. The amount of data was orders of magnitude less than normal. "
Dr. Olsen, the C.D.C. The epidemiologist pointed out that vaccine selection is based on more than just existing strains. Scientists are also considering other data, including projections about the likelihood of emerging groups of influenza viruses becoming more common in the coming months.
And the uncertainty about the return of influenza does not make it any less important to get vaccinated against the flu.
There's another hard-to-predict factor that could play an important role in the return of the flu: whether society will continue the pandemic-learned behaviors that benefit public health. Is wearing masks becoming the norm? Will employers give their workers more physical space?
Dr. Baker pointed out that the last time Americans had the chance to incorporate these behaviors into the culture without doing so.
"The 1918 influenza pandemic should have given us some kind of social learning," said Dr. Baker, but the behavior didn't change. "So what is the journey you will be making along this axis from the Covid-19 pandemic?" She added. "Are you going to wear your mask even if no one else is?"