NEW YORK — Flu season is off to its earliest start in nearly 10 years — and it could be a bad one.

Health officials on Monday said suspected flu cases have jumped in five Southern states, and the primary strain circulating is one that tends to make people sicker, especially the elderly.

“It looks like it’s shaping up to be a bad flu season, but only time will tell,” said Dr. Thomas Frieden, the director of the Centers for Disease Control and Prevention.

The good news is the nation seems to be fairly well prepared, Frieden said. More than a third of Americans have been vaccinated, and the vaccine is well matched to the strains of flu seen so far, CDC officials said.

Higher-than-normal reports of flu have come in from Alabama, Louisiana, Mississippi, Tennessee and Texas. An uptick in flu cases like this usually doesn’t occur until after Christmas.

Flu-related hospitalizations are rising earlier than usual, and there have already been two flu-related deaths in children.

It’s not clear why the flu is showing up so early.

The last time a typical flu season started this early was the winter of 2003-04, which proved to be one of the most lethal seasons in the past 35 years, with more than 48,000 deaths. The dominant type of flu back then was the same one seen this year.

One key difference between then and now: In 2003-04, the flu vaccine was a poor match to the flu strain.

Another: There’s more vaccine now, and flu vaccination rates have risen for the general public and for key groups such as pregnant women and health care workers.

In all, an estimated 112 million Americans have been vaccinated so far, the CDC said. Flu vaccinations are recommended for everyone who is 6 months of age or older.

On average, about 24,000 Americans die each flu season, according to the CDC.

Flu usually peaks in midwinter. Symptoms can include fever, cough, runny nose, head and body aches, and fatigue. Some people also suffer vomiting and diarrhea, and some develop pneumonia or other severe complications.