After two difficult Covid winters, the current season of respiratory sickness already rivals some of the worst cold and flu seasons on record — and it started about two months early.

R.S.V., or respiratory syncytial virus, has made so many young children ill this fall that weekly pediatric hospitalizations for R.S.V. are the highest recorded. Influenza, which normally peaks in February, has driven up hospitalization rates to the highest level for this time of year in more than a decade, surpassing hospitalizations from Covid-19. And while Covid illness is lower than it was the last two Decembers, it, too, is climbing.

Public health officials have been warning for weeks that a “tripledemic” of Covid-19, flu and R.S.V. would strain an already weary health care system. Hospitalizations from the three viruses have been rising together. Nationally, R.S.V. appears to have peaked, and flu is peaking in a few parts of the country, but infections from the two viruses are expected to plateau at high levels.

Experts say it is difficult to estimate the severity of the rest of this season because the coronavirus pandemic disrupted somewhat predictable patterns for other respiratory diseases.

“There’s a whole lot of winter left,” said Richard Webby, a virologist at the St. Jude Children’s Research Hospital in Memphis. “Certainly there’s lots of time for another Covid wave, and even enough potentially for another version of flu.”

The country has already faced two record-breaking seasons under Covid, which disproportionately affected older Americans, but the return of R.S.V. and flu this year means that some of the burden of illness has shifted to the country’s youngest — and their families.

Weekly hospitalizations for R.S.V. among children are the highest they have been since the Centers for Disease Control and Prevention began its surveillance in the 2018-19 season. Roughly one in every 70 babies 6 months and younger have been hospitalized since the beginning of October, according to preliminary estimates.

With flu surging and Covid-19 circulating, respiratory illness has overwhelmed pediatric units across the country, shifting the strain to emergency rooms and children’s hospitals.

“You ask people who are involved with either emergency services or hospitalizations and they’ll tell you this is the worst season that they can remember,” said Dr. Daniel Rauch, chief of pediatric hospital medicine at Tufts Medical Center.

“We are pretty scared for the winter,” he added. “I don’t know that our staff can keep it up.”

R.S.V. cases and hospitalizations appear to be peaking — particularly in the South, where the illness arrived first — but some experts predict they will level off and remain high for some time.

R.S.V. hospitalizations for older adults are also far higher than recorded at this time of year in past seasons.

The oldest Americans remain extremely vulnerable to severe illness from Covid-19 and flu, and with the flu’s early comeback and dramatic rise, public health officials are worried about this age group.

“Covid has not gone away,” said Dr. Fiona Havers, an infectious disease specialist at the C.D.C. “Hospitalizations, particularly in older adults and people with high-risk conditions, are still happening at high rates.”

Flu hospitalizations among the elderly are expected to increase in the coming weeks as families continue to travel and gather indoors for the holidays. The predominant type of flu circulating right now, a subtype of influenza A known as H3, also tends to result in higher flu hospitalizations among the elderly, according to the C.D.C.

The agency estimates that there have been at least 150,000 hospitalizations and 9,300 deaths from flu alone so far this season. It has also reported 30 deaths among children from the flu, a fraction of the 199 pediatric flu deaths estimated in the 2019-20 season.

Experts say that the available vaccines for flu and Covid-19 are good matches for the strains that are circulating. That means the shots should offer some protection against infection, though they are most effective at protecting against severe disease. For those who have already had the flu, the shot can protect against another strain they haven’t been exposed to.

But vaccination rates are low across the country. Just 36 percent of people 65 and older have gotten an updated Covid-19 booster this fall, and the rates are lower for all younger age groups. About 15 percent of adults 65 and older, and about 46 percent of children, have received the flu vaccine.