Britain is braced for one of the worst flu seasons for years, exacerbated by a lack of exposure during the Covid-19 pandemic.
A campaign has been launched, urging millions to get vaccinated for flu and Covid-19 after experts said they were increasingly worried about a serious surge this winter.
Officials at the UK Health Security Agency say there will be lower levels of natural immunity this year after a few winters when people socialised far less because of the coronavirus pandemic.
One expert said they were more worried about flu this year than they have been for several years.
The UKHSA warned of a possible “difficult winter” ahead as respiratory viruses, including flu and Covid-19, circulate widely.
International surveillance shows the UK can expect the spread of H3N2 (a subtype of influenza type A), which is currently the most commonly detected flu virus worldwide.
H3N2 has recently caused waves of infection in countries including Australia, which has just had its winter. The vaccine used by the UK is designed to fight this strain.
In 2017/2018, the H3N2 flu strain led to a severe UK flu season, with about 20,000 deaths and 40,000 hospital admissions.
It did circulate in the UK last winter but less social mixing because of Covid-19 means there is little immunity to it.
Under plans announced on Wednesday, about 33 million people in England will be eligible for a free flu vaccine this year, including all primary school-age and some secondary school-age children, who will be offered the nasal spray.
About 26 million people in England are also eligible for the autumn Covid-19 booster vaccine.
The UKHSA said that, in addition to a predicted flu wave, there are “early indications” that Covid rates are beginning to rise as winter approaches.
Dr Susan Hopkins, chief medical adviser at the UKHSA, said: "Flu and Covid-19 are unpredictable but there are strong indications we could be facing the threat of widely circulating flu, lower levels of natural immunity due to less exposure over the last three winters and an increase in Covid-19 circulating with lots of variants that can evade the immune response.
“This combination poses a serious risk to our health, particularly those in high-risk groups.
“The H3N2 flu strain can cause particularly severe illness. If you are elderly or vulnerable because of other conditions you are at greater risk, so getting the flu jab is a sensible, potentially life-saving thing to do.
“We are extremely fortunate to have vaccines against these two diseases.
“Most eligible groups have been selected because they are at higher risk of severe illness.
“Younger children are unlikely to have built up any natural immunity to flu and therefore it is particularly important they take the nasal spray vaccine this year.
“So, if you are offered a jab, please come forward to protect yourself and help reduce the burden on our health services.”
England’s deputy chief medical officer, Dr Thomas Waite, said: “Vaccines have saved many lives over the years for both flu and more recently Covid.
“But we must not be complacent ― infections will rise once again this winter, so it’s really important people get both their Covid and flu vaccines if eligible.”
The NHS director for vaccinations and screening, Steve Russell, said: “This winter could be the first time we see the effects of the so-called ‘twindemic’ with Covid and flu in full circulation, so it is vital that those most susceptible to serious illness from these viruses come forward for vaccines to protect themselves and those around them.”
NHS Providers said trust leaders are “bracing themselves” for the possible “twindemic”.
The organisation’s director of policy and strategy, Miriam Deakin, said: “The recent uptick in Covid-19 cases and hospital admissions is concerning and time will tell if this ripple turns into a full-blown wave.
“Covid-19 boosters and flu vaccines this autumn will reduce the risk of serious illness and we would urge everyone to have their jabs when they can.”
People who qualify for both jabs could be offered the flu and Covid jab at the same time if supplies allow, although in different arms.
Those eligible for the flu jab are: people aged 50 and over; those aged six months to 49 with a specified health condition; secondary school-aged children focusing on Years 7, 8 and 9 with any remaining vaccine offered to Years 10 and 11; primary school-aged children; pregnant women; those in care homes, frontline health and social care staff; carers and the household contacts of people with weakened immune systems.
These people can get a jab from their GP surgery or pharmacies offering an NHS vaccine service.
GPs are also inviting children aged two and three years old (as of August 31) for the nasal spray vaccine.
People who qualify for the autumn/winter Covid booster include: adults aged 50 and over; those aged five to 49 with health conditions that put them at greater risk; pregnant women; care home workers and frontline health and social care workers; carers and the household contacts of people with weakened immune systems.
All those eligible for a booster can now book online, apart from people aged 50 to 64 (who are not in an at-risk group), who will be able to get one later this autumn.
People with asthma are not routinely offered a Covid booster vaccine owing to a lack of evidence that they are at higher risk from the virus.
But a subset of people with asthma (such as those with very poorly controlled asthma) are offered a dose.