The flu season in the United States has started earlier than usual this year, with infections spreading noticeably faster and case numbers rising, particularly in southern and western states.
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In its latest report on Friday, the US Centers for Disease Control and Prevention (CDC) said seven states recorded high levels of flu activity last week, compared with just two the week before.
The CDC said more than 3% of patient visits to doctors' clinics last week were linked to flu-like symptoms, indicating that the season has entered a period of real spread. Louisiana is the worst-affected state so far.
Dr Fred Lopez, an infectious disease specialist at the Louisiana State University Health Sciences Center in New Orleans, said the flu season is picking up pace and the upward trend is likely to continue over the next month or two.
"If you have not yet had the vaccine, get it now," he said.
The CDC report noted that flu activity at the start of the current season is above the usual level for this time of year, but it is not yet clear whether this means the season will be more severe than previous ones.
In a worrying sign, Dr Anna Durbin of Johns Hopkins University said during a recent media briefing that the rate of flu hospitalisations in mid-October was the highest recorded so far in such an early part of autumn in more than 15 years.
The CDC estimates that since the start of the current season, flu has caused at least 2.6 million illnesses, 36,000 hospitalisations and 2,100 deaths.
The outbreak began in the south and west of the country. According to CDC data, states that have so far recorded high flu activity include California, Louisiana, Oregon, South Carolina, Tennessee, Texas and Washington.
Early flu waves in Louisiana are not unusual, as the state is often among the first to see a rise in cases each year.
Lopez explained that this is partly due to lower vaccination rates, higher levels of poverty and the difficulties residents face in accessing healthcare.
Warnings of more infections over Thanksgiving
Doctors expect flu and other respiratory viruses to spread faster later this month as travel and family gatherings increase, coinciding with the Thanksgiving holiday in the United States.
Dr Nicole Iovine, an infectious disease specialist at the University of Florida, said in a statement emailed this week: "As we prepare to gather with family and friends over the coming holidays, getting the vaccine becomes extremely important."
The report said vaccination should be postponed for people who currently have severe symptoms, because the immune system is busy dealing with the illness.
Nevertheless, Lopez stressed the importance of receiving the flu vaccine even for people who were infected earlier this autumn. He explained that the vaccine protects against three types of influenza viruses, and that the spread of more than one strain in a single season is not unusual.
Last winter's flu season saw a sharp rise in cases in late December, with severe spread in some parts of the United States. Health officials in New York City described that season at the time as the most severe in 20 years.
Infections began to decline in the following months, and the season effectively ended by late March.
However, in the spring the CDC estimated that the season had caused at least 32 million illnesses, 380,000 hospitalisations and 24,000 deaths.
A mutated strain behind most infections
Data showed that most flu cases recorded this autumn are caused by a version of the influenza A(H1N1) virus, which carries genetic mutations that may give it a greater ability to evade the immune system's defences.
Nevertheless, the data available so far suggest that this season's vaccine can recognise and respond to the circulating virus, according to Andrew Pekosz, an immunologist at Johns Hopkins University, speaking at a recent media briefing.
Pekosz said health officials will not be able to determine precisely how effective this year's flu vaccine is until several months have passed.
"At the moment, there is nothing to be alarmed about," he said.
