Flu continues to spread across the country, but the severity of illness may be going down in some regions, officials at the Centers for Disease Control and Prevention said Friday.

In its weekly update on influenza, officials pointed to several flu activity measures that continued to rise and others that fell.

“The bottom line is, it’s flu season, and most of the country is seeing influenza activity or has seen it and it may continue for a number of weeks,” CDC Director Tom Frieden said.

Flu is notoriously hard to predict. The dip in flu severity in some parts of the country — the South and Southeast — may be because fewer people went to the doctor during the holidays, officials said.

Each year, there tends to be an increase in flu activity towards the end of the year, followed by “a little notch downward,” said Joseph Bresee, chief of epidemiology at CDC’s influenza division. “I wouldn’t be surprised if the curve went up next week.”

This flu season is having its earliest start in nearly a decade.

CDC officials also said preliminary data show this year’s flu vaccine to be about 62 percent effective, or moderately effective, in line with years past. That means someone who received the vaccine was 62 percent less likely to go to the doctor to be treated for flu.

This year’s season started about a month earlier than normal and has hit hard. Higher-than-anticipated demand for vaccine has caused spot shortages across the country and made vaccine hard to find in the Washington area and elsewhere. Several area doctor’s offices ran out of vaccine Friday, according to physicians and patients.

Flu was widespread in 47 states last week, up from 41 the week before, according to the latest CDC data for the week ending Jan. 5. The only states without widespread flu are California, Mississippi and Hawaii.

The number of states experiencing high levels of influenza-like illness fell to 24 from 29 the previous week, the CDC said. But more states reported moderate flu-like illness.

The proportion of death certificates that include “pneumonia or influenza” as the cause of death rose to 7.3 percent last week. That is just over the official “epidemic threshold” of 7.2 percent, but is not unusual for this time of year.

CDC officials did not provide a total number of deaths from flu, but tens of thousands of Americans die from it or its complications each year.

The flu season runs from late September through late May, and influenza typically peaks in January or February, though sometimes later. In early December, the CDC said it was noticing an uptick in flu activity about a month before authorities typically see it.

Even though vaccine may be harder to find and the vaccine is “far less than we wish it would be,” Frieden urged those who haven’t done so to get vaccinated. “It’s still the best tool by far we have to prevent the flu.”

Unlike vaccines for many childhood illnesses that have more than 90 percent efficacy and last longer than a year, flu vaccine has to be reformulated every year.

Flu vaccine effectiveness has ranged from 50 to 70 percent over the past two decades, Bresee said.

“Flu vaccines are tough,” Bresee said during a telephone briefing with reporters. The virus mutates often, and the antibodies that people produce only protect them for one flu season.

And the groups who are most vulnerable — young children, those over 65 and people with compromised immune systems — are also the people for whom the vaccine may work less well because they don’t produce adequate levels of antibodies, officials said.

Flu vaccines are designed to protect against three influenza viruses that experts predict will be the most common during the upcoming season. The three viruses that commonly circulate among people today are influenza B, influenza A (H1N1) and influenza A (H3N2). Each year, one flu virus of each kind is used to produce seasonal influenza vaccine.

The primary strain this season is H3N2, an influenza A virus that has been associated with more severe flu seasons. The vaccine formulated for this 2012-2013 flu season year is well-matched to the strains of the virus seen so far, officials said. It contains two strains of influenza A and one strain of influenza B.

This season’s vaccine “has space for only three strains,” Frieden said, but next year, some vaccines will be designed to protect against four flu viruses — two influenza B and two influenza A.