Originally Published: 17 AUG 23 07:31 ET

By Jen Christensen, CNN

(CNN) — Last fall, children’s hospitals across the country dealt with an unprecedented early surge of patients with respiratory syncytial virus, known as RSV. By October, there were so many sick kids needing care that some states declared states of emergency, and some facilities set up tents with extra beds in parking lots.

This year, experts think the timing of the RSV season will be closer to the pre-pandemic normal, but case numbers will probably be anything but — that is, if people embrace newly available tools to prevent RSV.

Why a cold-like disease can be so severe

RSV is ubiquitous. Nearly everyone has been infected with the virus before age 2, the US Centers for Disease Control and Prevention says. Infection usually causes symptoms that look a lot like a cold: a runny nose, a cough, sneezing, wheezing and possibly a fever.

For most healthy adults and kids, RSV is a mild infection — but it’s unpredictable. A study published Tuesday found that most kids who were in the ICU with RSV last year had no underlying health conditions and had been born full-term.

Young infants and the elderly are particularly vulnerable to more severe RSV infection. They can become dehydrated and develop breathing trouble, and the infection can turn into more serious illnesses such as pneumonia and bronchiolitis, inflammation of the tiny airways in the lungs.

The CDC does not track hospitalizations or deaths for RSV like it does for the flu, but for babies who need hospitalization in the first year of their lives, studies show, RSV is the leading cause.

Each year, the virus leads to hospitalization for up to 80,000 kids under the age of 5 and up to 160,000 people older than 65. It kills between 100 and 300 children and between 6,000 and 10,000 older adults every year, according to CDC estimates.

Virus seasons out of whack

RSV is usually a seasonal disease, the CDC says, causing illnesses in the US between October and April, with the largest wave coming in December or January. But the Covid-19 pandemic threw everything off.

The 2020-21 RSV season had a historically low number of RSV cases.

The virus spreads when someone coughs or sneezes near you, or their respiratory droplets contaminate surfaces like doorknobs or elevator buttons. Scientists think lockdowns and other steps people took to avoid Covid, like masks and frequent handwashing, helped keep RSV away.

The lower RSV case counts during the pandemic created a kind of “immunity gap,” some scientists say. With the 2021-22 season, case counts were closer to pre-pandemic levels, and the virus started spreading in earnest in May, about 21 weeks earlier than usual. That season peaked in July and ended early, in January.

In 2022-23, RSV season started a little earlier than usual, in June, but it came back with a vengeance. Kids who would ordinarily have caught the virus during the pandemic years had no immune familiarity, and they started flooding hospitals in November.

Older adults also filled hospitals. The rate of RSV hospitalization for people 65 and older was 10 times higher than usual by November 2022, according to the CDC.

Typical timing expected

Things should be back to normal in the US this year as far as timing, according to Dr. Daniel Weinberger, an associate professor in epidemiology of microbial diseases at the Yale School of Public Health.

“It seems like we are going to have RSV during the RSV season, which has been difficult to say over the last few years,” he said.

Much like with the flu, scientists looking for a sense of where this season may go watch the Southern hemisphere, where it’s now winter and the RSV season is just about wrapped up. It’s not always a close predictor of what’s going to happen in the US, but it can help.

Weinberger said it seemed like this season kicked off close to its pre-pandemic timeframe, or slightly early in some countries.

With so many people ill with RSV in the US last year, he says, the intensity of this year’s season will probably be “closer to normal” and more in keeping with pre-Covid numbers.

Dr. Claudia Hoyen, director of pediatric infection control at UH Rainbow Babies and Children’s Hospital in Cleveland, said that although they were already seeing RSV cases at this point last year, this summer has been different.

Hoyen said she gets a daily report of “who is positive for what,” and the past three months have been “eerily quiet.”

For pediatricians who live by disease seasons, the past few years have been disconcerting, since “there’s been no season to anything.”

“Hopefully, it’s not the quiet before the storm,” she said.

New vaccines, antibody available

One factor that might improve things this season is a newly approved monoclonal antibody to protect babies against RSV. One of the main reasons scientists try to anticipate waves of seasonal infections is so they can get proactive therapies or vaccines in place.

This year, for the first time, there are vaccines for older adults and a preventive for all infants. Knowing when to use them makes them more effective.

Since 1998, a single monoclonal antibody has been available to provide protection against RSV, but it was only for high-risk infants. The highly effective treatment is given once a month during RSV season.

Now, infants and young toddlers will have another option to protect them against RSV. In July, the FDA approved nirsevimab, marketed as Beyfortus, a long-acting monoclonal antibody that the CDC recommended for all infants younger than 8 months who were born during RSV season or who are entering their first RSV season and for children between 8 and 19 months who are at higher risk of severe RSV. In trials, the treatment reduced the risk of hospitalization and doctor visits for RSV by about 80%.

There are also two RSV vaccines available for people 60 and older. Both are single-dose vaccines that the CDC recommended in June. Their protection should last at least a few seasons, so doctors are recommending that older people get them when they get their flu shot.

“It does have the potential to be hugely impactful,” Weinberger said. “If people use these interventions, they could really make a big dent.”

When the monoclonal antibody treatment came out for vulnerable infants in the ’90s, Hoyen said, it was transformative.

“Before they were available, RSV season was an absolute terror for us, because we knew that with each season, there were going to be some babies that didn’t make it,” she said. “Once we got that monoclonal antibody treatment, it saved so many lives. It’ll be interesting to see what the uptake is, but it could clearly help save the lives of babies.”

How many adults and children will actually get that protection against RSV remains to be seen. The number of parents who got their children vaccinated against Covid-19 remains low, but there might not be as much hesitation with a monoclonal antibody, experts said.

The American Academy of Pediatrics sent a letter to the head of the CDC this month encouraging the US to develop a comprehensive strategy to ensure equitable access to the therapy.

There has been a lack of general awareness about RSV, surveys show, but more people may be aware now because of the dramatic images of overwhelmed hospitals last year.

“If there is perhaps the one upside of last year being a severe season is that I think a lot more people are aware of the dangers of RSV for infants and older adults than they previously were,” Weinberger said. “Hopefully, that will encourage people to get vaccinated or get the antibodies going into the season this year.”