Flu season is not over yet but we have already started wondering about the efficacy of the most effective weapon against influenza this season (2018-19).

Before we look into the efficacy, a few words about flu and flu vaccine. Skip next 3 blocks if you want to go the efficacy directly.

Influenza is an acute viral respiratory tract infection caused by two types of influenza viruses – type A and type B (there are type C and D, but they are not considered a significant threat to our wellbeing).

Influenza can spread from human to human by droplet, aerosol (generated by cough) and direct contact with the respiratory secretion.

The influenza virus may not lead to any symptoms in some individuals but it usually causes sudden onset of fever, chills, headache, muscle pain, fatigue and rare but often fatal pneumonia. Healthy individuals usually recover within two to seven days. However, a certain group of people are prone to complications (bronchitis, secondary bacterial pneumonia, otitis media, meningitis, encephalitis ). These are children under six months of age, older people, underlying health conditions like respiratory or cardiac disease, chronic neurological conditions, immunosuppression and pregnant women.
Pregnancy-associated complications are increased perinatal mortality, prematurity and low birth weight.

The mainstay of management of influenza is prevention by yearly vaccination especially to the vulnerable groups of people, healthcare professionals, appropriate infection control precautions and antiviral drugs oseltamivir, zanamivir (and newer drugs – peramivir, baloxavir). These drugs are only effective if taken within a certain period of onset of symptoms and must be assessed and prescribed by a healthcare professional.

Flu virus EM
Influenza virus under electorn Microscope (CDC/PHIL/ Dr. Terrence Tumpey/  #8160 )
Flu virus 3D model
A 3D model of influenza virus (Wikimedia commons)

Influenza vaccine available in the UK are made of either inactivated (killed) virus or attenuate (weakened, stripped of their disease making property) virus. So they can not produce influenza upon vaccination (unless we have ‘the night king’ nearby to raise the dead virus)!

However, it is possible to have a flu-like illness after the vaccine, because our immune system recognises the dead/inactivated virus particles and start preparing our defences against the similar virus in future (which is the live flu virus of course). If this immune response is stronger in a person, then they may have symptoms like flu.

It might be interesting to know that most symptoms we get from influenza infection are a result of our immune system fighting the virus rather than the direct assault of the virus on our body. Post-vaccine symptoms are also the effect of our immune system, and not flu, there is no live virus involved.

However, vaccine-induced immunity takes about two weeks to develop. So if someone catches a flu virus within this period, they might contract flu in spite of having the vaccination.

So why we need a vaccine every year? It is because of the flu virus can change (mutate) frequently and also can mix and match with other flu viruses. These changes, however, give the virus and unique advantage. Our immune system, which developed a capability to deal with the virus after its vaccine training fails to identify the new/mutated virus, hence the vaccination only provide a short lasting immunity until the virus changes.

The virus changes so frequently that the vaccine needs to be adjusted every year to cover the most prevalent circulating flu virus. This difficult job is done by the World Health Organisation (WHO). They gather the data from all over the world and tries to predict which type of virus will likely to cause flu in the forthcoming year. They are very good at predicting but it is possible that a new virus emerges after they have made the prediction and vaccine production started. Mismatches between the vaccine and circulating viruses do occur from time to time explaining the fall in effectiveness. WHO includes three/four strains (types) of viruses in the vaccine to cover most of the circulating virus.

2019-20 vaccine (Northern hemisphere) will have
Flu A/Brisbane/02/2018 (H1N1)pdm09-like virus
Flu A/Kansas/14/2017 (H3N2)-like virus
Flu B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage)
The 4th – for the quadrivalent vaccine
Flu B/Phuket/3073/2013-like virus (B/Yamagata lineage)

2018-19 vaccine (northern hemisphere) has
Flu A/Michigan/45/2015 (H1N1)pdm09-like virus
Flu A/Singapore/INFIMH-16-0019/2016 A(H3N2)-like virus
Flu B/Colorado/06/2017-like (Victoria lineage) virus
The 4th – for quadrivalent vaccine
Flu B/Phuket/3073/2013-like virus (B/Yamagata lineage)

How effective was the vaccine?

The UK (Data from the Eurosurveillance paper, by Esther Kissling et al)

The Eurosurveillance paper found that the most common flu virus infecting patient this year was Flu A and the majority of the flu A virus was H1N1 (pdm 2009) type.

Patient groupEfficacy
In the age group 18-65 years37%
In children who received the quadrivalent live attenuated nasal vaccine (in the UK)80%
In the target population (sample from multiple European Countries)59%
The vaccine efficacy in primary care/community in all age group combined43%

So influenza vaccine was effective in reducing the risk for medically attended influenza illness up to 80% in children.

It is much better compared to the 2017-18 season. Public Health England reported:
flu vaccine was
15% effective in all age groups,
26.9% effective in children aged 2 to 17 years,
12.2% in at-risk groups aged 18 to 64 years.

USA (Data from Interim Estimates of 2018–19 Seasonal Influenza Vaccine Effectiveness — United States, February 2019)

Patient groupEfficacy
overall adjusted vaccine effectiveness against all influenza virus infection47%
For children aged 6 months–17 years61%
vaccine effectiveness against influenza A(H1N1)pdm09 (overall)47%

Vaccination remains the best method for preventing influenza and its potentially serious complications.
People who do not develop symptoms still shed the virus-laden particles during tidal breathing, which has the potential to cause infection to others.

During past seasons, including the 2017–18 season, approximately 80% of reported pediatric influenza-associated deaths have occurred in children who were not vaccinated. Vaccination also has been found to reduce the risk for influenza-associated hospitalization in pregnant women and can reduce the risk for cardiac events among persons with heart disease (CDC).


Interim Estimates of 2018–19 Seasonal Influenza Vaccine Effectiveness — United States, February 2019,



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