Flu is striking earlier than usual in England, experts have said, insisting that vaccination is the best way to blunt its impact.

According to the UK Health Security Agency (UKHSA), flu is at low levels, but various metrics, such as the proportion of positive tests and GP consultations for influenza-like illness, suggest it is on the rise, with the number of emergency-department attendances for influenza-like illness above seasonally expected levels.

“Flu activity throughout September has been higher than in the same period during all past four winter seasons,” UKHSA reported, adding that flu trends had been rising since August.

Prof Sir Andrew Pollard, the director of the Oxford Vaccine Group, said: “There is one simple action for all those who are at risk because of age or their underlying health conditions, which is to get vaccinated as soon as possible and before we reach the peak of transmission.

“Everyone can benefit from being vaccinated but free vaccines are only available for children and those in higher-risk groups.”

He added that those who were not eligible for a free flu vaccine via the NHS might be able to access them for free through their workplace, while vaccines were also available privately for anybody able to pay for them.

A private flu vaccination costs about £20 at most high street pharmacies, and some retailers offer them for under £10 as part of membership schemes.

Pollard said an earlier flu season did not necessarily mean a worse flu season. “We don’t know for certain why the timing varies each year.” He believed an early rise was due to a combination of factors, including the weather, the particular strain of the virus that is around and its ability to spread, the immunity of the population to the circulating strain, and aspects of behaviour – for example, the number of close contacts people have.

Antonia Ho, a professor of infectious diseases at the University of Glasgow’s Centre for Virus Research, said it was important to get vaccinated as soon as possible as it took a few weeks for the vaccine to provide full protection.

“An early rise in influenza cases may indicate a more intense influenza season overall, because earlier onset is associated with a longer period of community transmission and higher cumulative case numbers,” she said. “However, onset timing alone doesn’t determine severity.”

Data from UKHSA has suggested the increase in flu is being driven by cases among young people aged 18 to 24, and that a mix of two flu A subtypes – H1N1 and H3N2 – is circulating in England. “The flu vaccine covers both subtypes and also influenza B,” said Ho.

While UKHSA has noted that the H1N1 subtype of flu A appears to have changed or “drifted” a little from the strain that was used to make this year’s flu vaccine, Ho said it was too soon to say if that would affect how well the vaccine worked.

According to data from UKHSA, the vaccination programme in England last winter is estimated to have prevented about 100,000 hospital admissions and 7,000 deaths, despite the flu season again starting earlier than usual.

John Tregoning, a professor of vaccine immunology at Imperial College London and author of the book Infectious: Pathogens and How We Fight Them, said: “Last year is a good example where [we] also had an early start to the flu season, and then because of that, probably people slightly changed their behaviours and went and got vaccinated, and then it comes down again.”

Experts have urged those eligible for a Covid jab to get vaccinated, with UKHSA data suggesting levels of Covid are rising. Tregoning also emphasised the importance of vaccination against respiratory syncytial virus (RSV).

Ho said that while vaccination against respiratory diseases was important, there were additional measures people could take if they started to feel unwell.

“If you have flu-like symptoms, ideally stay at home, practice good hand hygiene, avoid contact with others,” she said.