What we need to know about flu and its vaccines 

When the Covid-19 pandemic started, earliest thoughts were that Covid-19 could be considered like a version of flu, indeed much emphasis was given to the fact that the number of hospitalisations and deaths during a ‘bad flu season’, especially in the most vulnerable,  were known to be substantial.  Many authorities were slow to accept Covid-a9 was different!

Further, most of the early thoughts about transmission, and its prevention, were based on knowledge and evidence from research on flu.  Clearly Covid-19 is a much more serious disorder than flu, both in public health and individual clinical outcomes, but this winter there is current emphasis on a successful flu vaccination programme. 

In this post I consider what we know about flu and its vaccines and why these issues are especially important in relation to controlling the impact of Covid-19 

Flu and Influenza viral infections: ‘flu’

• For the sake of simplicity, I am going to separate out ‘flu’ the illness from the viral  infection ‘influenza’
• We all diagnose ourselves as having flu based on the combination of having a fever, muscle aches and pains and feeling lousy
• Like Covid-19, flu can cause severe complications such as pneumonia or heart failure but these are really restricted to those with underlying health problems  
• There is no diagnostic test but doctors and patients feel more confident about ‘flu’ being the cause of our symptoms when many others have a similar pattern
• Many viruses can cause these kinds of symptoms and indeed other infections such as a bacterial sore throat or glandular fever can mimic what we call flu
• It is easy to forget that in the pre-Covid-19 era, there was very limited data about the viral background to people with clinical symptoms.  There were no equivalents of lateral flow or PCR testing available for widespread use
• We don’t know how common asymptomatic infection was because this was not, and is not, routinely tested in flu
  • Flu does appear to have a very short incubation period of around 2 days and in that period people can pass the infection on but not be ill
  • How likely it is that people get infected and don’t have symptoms at all is not known and this will also vary each year, depending on the strain

Viral influenza

• The presumption is that many people with ‘flu’ symptoms have an infection with one of the specific influenza viruses
• There are 2 main types, amazingly called Influenza A and Influenza B!  (Actually there is Influenza C, which is rare, and Influenza D, which is only an animal infection)
• The clinical infections caused by Influenza A and Influenza B are very similar 
• There are also many different sub-types of both Influenza A and Influenza B 
• The Influenza A sub-types are determined by the presence on the virus surface of two different types of proteins: ‘H’ and ‘N’
• You may remember that the Covid-19 virus has the one very important spike protein on its surface, sso the Influenza A virus has 2 important spikes

• This is the complicated bit!
  • There are 18 different varieties of H spikes (H1-H18) and 11 different varieties of N spikes (N1-11) so in theory there are 198 (18*11) different combinations 
  • The two most common combinations are H1N1 and H3N2 and it is against these the vaccines are designed to protect us
  • Although infection with one type may give protection against one of the others, this is not often very strong and cannot be relied upon
• And even more complicated!
  • Each year different genetic mutations appear within each subtype such that for example a H1N1 circulating in one year may be quite different to a H1N1 three years later.  
  • To consider how this compares with the coronaviruses. 
    • Covid-19 is one subtype of coronavirus, equivalent to an Influenza A HN subgroup. So an infection with one coronavirus eg SARS does not give immunity against other coronaviruses eg Covid-19
    • But Covid-19 has developed many different variants, such as Delta, each with some differences on the spike protein.  Unlike Influenza A though, there is some protection carried through from one Covid-19 strain to another. We do not probably have this carry over protection with influenza variants
• Influenza B also has subtypes (to be strict they are called lineages ie from their origins) and these two lineages (called ‘Victoria’ and ‘Yamagata’) also have different genetic strains which change each year
• So that is why we need a new vaccine each year, because the virus strains can be so different with the result that our past exposures may give us little or no immunity 

What is in a flu vaccine ?

• The challenge with developing flu vaccines is based on the complexity and changing nature of the circulating strains
• We never know from one season to another how much immunity is carried forward
• Each year a bunch of experts from the World Health Organisation try and predict what strains the vaccines  will need to protect against
• The emerging vaccine actually normally has 4 separate vaccines  – two active against the presumed likely Influenza A strains of H1N1 and H3N2 and two active against the likely Influenza B strains of Victoria and Yamagata

• Interestingly the strains of interest are conveniently named after the place where the current strain emerged.
• So the 2021/22 vaccine contains vaccines against the following

Are the flu vaccines like the Covid-19 vaccines?

• The Covid-19 vaccines such as Pfizer and AstraZeneca are manufactured by very different approaches to the flu vaccines which are developed using traditional vaccine manufacturing methods
• The flu vaccines are principally of 2 types
  • live but very weakened forms of the 4 viruses, which can be given as a nasal spray
  • an inactivated form of the 4 viruses, some of which are grown on eggs, which require an injection
• Adults are always given the injectable forms.  
• There is always a debate as to which is better both in terms of side effects and effectiveness

Who should get flu vaccines?

• An interesting question in these Covid-19 times!
• Typically, national programmes target those above a certain age, such as 50, or those who are clinically vulnerable as the infection is much less likely to be serious in younger healthy adults 
• Key workers such as health care professionals are also targeted 
• Widespread use of flu vaccines to both infants and school age children is recommended, both to preserve schooling and also to help in reducing the spread of flu in the population.

• The fact that protecting children against flu, despite it being a mild disease for that group, is widely accepted – which is interesting given the great ongoing debate about protecting children with Covid-19 where similar arguments apply!

How effective are flu vaccines?

• With Covid-19 we have been wanting 90% or greater protection and the bar has been set high for being successful
• Interestingly, seasonal flu vaccines are much less protective BUT:
  • No-one tests flu vaccines for protection against having asymptomatic infection, so the effectiveness is only based on people reporting being unwell
  • Given what was said above, if someone develops a ‘flu-like’ illness despite being vaccinated it doesn’t mean that the vaccines didn’t work against the strains they were designed to protect 
• Below are data about the modest vaccine effectiveness from the last 15 years of flu vaccines in the USA  – but other countries will have similar results 

• The reasons why the vaccines are not as effective as we would like will depend on:
  • The effectiveness of the vaccines in providing immunity against the specific strains that have been targeted
  • Whether the circulating strains are different from the ones that have been targeted 
  •  What natural immunity from previous flu vaccines and illnesses may be carried forward
• Why are we not given two flu vaccines if one is not that effective?

• The short answer is a second shot has not been shown to give additional benefit
  • The studies that have been done have been on at-risk groups such as people with cancer or on dialysis, so maybe people who are otherwise well might also benefit from two shots
  • There is a health economic argument against two shots for healthy people for a disorder that otherwise is very self-limiting 

Conclusion about flu in the era of Covid-19

• Below is a headline from 2018 from the UK: flu is not an insignificant public health problem

• Much of the original thinking about tackling Covid-19 came from what we knew was important for managing flu
• This includes issues such as the benefits of masks, hand washing, ventilation etc
• In 2018 and 2019, there were just under 30,000 deaths from flu and its complication in the UK
• Up to now we did not have the tools to test for viral infections such as flu, so the true size of the problem was unknown
• In most western countries handwashing or mask wearing, whilst effective against flu were never taken up as important public health measures 
• What the Covid-19 pandemic has achieved maybe is to our taking flu more seriously because of the risk of spreading to the most vulnerable and also none of us like being laid low year after year!

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