Could flu be a bigger problem than Covid this winter? Professor Anthony Harnden, the deputy chair of the Joint Committee on Vaccination and Immunisation, has warned that it might be, suggesting that the low prevalence of flu over recent months could come back to ‘bite us’ as the weather worsens. There are also fears that reduced levels of flu in recent months could make it much harder to develop a successful jab.

In a normal year, the route to a flu vaccine is well trodden. The annual flu vaccination programme first began in England in the 1960s, and since 2000, all over 65s have been offered the jab every year. Healthy children have also been offered a live attenuated influenza vaccine (LAIV) in school, administered as a nasal spray, for the last eight years.

But because developing the flu vaccine effectively involves a sophisticated guessing game, this will be a much more difficult process this year. Flu is an intelligent virus which mutates very quickly and has many different strains; the vaccine must be tweaked annually to try and ‘catch’ the latest variants of concern. Every year, scientists receive flu data from the other side of the world. They use this to predict which strains are the deadliest and most likely to infect their populations. Around now, scientists in the Northern Hemisphere will tweak their jabs based on data from the flu season in the Southern Hemisphere. There’s a problem this year though: there isn’t much flu.

Each year, public health labs perform genome sequencing and analysis on 7,000 influenza viruses on clinical samples collected through flu surveillance from around the world. The WHO publishes a report every February on their recommendations for the influenza vaccines for the Northern Hemisphere based on the previous year’s data. This year’s report is rather stark.

‘Overall, record-low levels of influenza detections were reported and fewer viruses were available for characterisation during the September 2020 to January 2021 time period than in previous years,’ the report says. Clearly, 2020 and 2021 have been abnormal years: whether it is because of social distancing measures or because Covid-19 may have replaced flu as the dominant seasonal illness, flu has effectively been eliminated, at least in the short term, from many countries.

According to the WHO, in comparison with previous years, there has been a 99 per cent reduction in influenza positive samples in the Northern Hemisphere. NHS data paints a similar picture. Only 40 hospitalised flu cases were reported from 56 NHS trusts across England during the most recent flu season, a number which appears to be far lower than during a ‘normal’ year.

The data from the Southern Hemisphere’s inter-season, which is essential to vaccine development in the Northern Hemisphere, is also minimal. From around 60,000 specimens tested in the Southern Hemisphere, only 11 cases of flu were recorded, originating from influenza A or B. Not a single case of flu was recorded in Australia or New Zealand, however cases of the childhood illness, flu-like Respiratory Syncytial Virus (RSV), have increased significantly in 2021, possibly due to children lacking general immunity.

This presents a significant challenge to scientists, where limited data limits their choices for flu variants of concern. The vaccine is always a gamble, but the stakes are much higher this year: levels of natural immunity are low, and there may be pressure on health services from Covid-19. The double-whammy of Covid and flu is an alarming one.

Sometimes, flu strains appear out of nowhere, emerge too late for vaccine development, or prove more infectious than previously thought: this happened most recently in 2017-18, where the vaccine was reported to be only 10.1 per cent effective in preventing GP visits in those aged 65 and over, the most at-risk group. Over 50,000 excess deaths were reported that winter, the highest on record since 1976.

A further complication occurs as vaccine trials are only ever done in healthy young people, so efficacy in older people is unknown even in a good year.

The government is clearly worried about the flu. The inoculation programme will be the largest ever attempted in the UK, as the number of eligible people is expected to be expanded and millions of pounds is poured into the programme. So will this year’s flu outbreak be much worse than normal? Nobody quite knows. For now, it is something of a guessing game.
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