It’s that flu time of year again

Now that the Christmas and New Year festivities are over, everyone is returning to work and gearing up for another winter. At this time of year, two of the main – and related – issues at the forefront of the NHS are coping with the winter and coping with flu. It has been recently reported that there was a sharp rise in hospital admissions with confirmed flu in the last weeks of December 2017. Accordingly, there remains a drive to ensure that people are vaccinated. GPs aim to vaccinate with the Influenza vaccine as many adults over 65 years old as possible, as well as the routine flu vaccination of pregnant women, children between the ages of 2 and 8 and those at risk of catching the virus (patients with chronic lung or heart disease, for example). In addition, the NHS tries to vaccinate as many of its healthcare workers in the frontline as it possibly can.

The Influenza virus is quite complicated and there are many types. In a nutshell, though, it can be subdivided into Influenza A and B, although there is also a very mild childhood type called Influenza C and a clinically irrelevant Influenza D. Influenza A is the most complex and most virulent variety: it is subdivided according to the proteins that it has in its capsule, known as H and N proteins. This broad classification is how Influenza A is described and how you will see the virus referred to, for example, H3N2, which describes a particularly virulent form. Presently, there are about 18 known H proteins and 11 known N proteins on the surface of the virus, many of which are not found in “strains” that infect humans but which are in birds and other animals (e.g. pigs). This H and N classification is yet further subcategorised into more specific forms of the virus e.g. A/Hong Kong/4801/2014-like virus is a strain of the H3N2 virus which will be included in the present 2017/18 vaccine, as will be the B/Brisbane/60/2008-like virus which affects children. The H1N1 virus was responsible for the Swine-flu pandemic in 2009 (A(H1N1)pdm09). Influenza B is milder than A and does not have the same categorisation as A.

Influenza is highly infectious with an incubation period of 1-3 days. Transmission is via aerosol droplets, so it can spread rapidly. Viral “shedding” (when people are still infectious) tends to occur from day 1 to 5-7 days after. Cases usually occur within a 2-3 month period in the winter, although it is never certain exactly when that period will begin. The “flu season” is considered to run from October to March or April (in the Northern Hemisphere), but most cases will occur between December and February. Symptoms include sudden onset fever, chills, headache, myalgia (aching muscles) and fatigue. There may be associated common cold symptoms, such as a dry cough, sore throat and stuffy nose. In most people, recovery can be expected in 2-7 days, but there can be secondary complications, including bronchitis, pneumonia and ear infections. Occasionally, severe infections can lead to meningitis or encephalitis. It can kill.

Flu is a worldwide problem. In the last century there were three pandemics: 1918 (Spanish flu), 1957 and 1968. The 1918 pandemic is estimated to have killed about 40-50 million people worldwide. In this century, the first pandemic was in June 2009, caused by the A(H1N1)v virus. Every year, however, there are worldwide about 3-5 million cases of severe flu with about 250,000 to 500,000 deaths (according to the World Health Organisation). These are frightening statistics. It is difficult to ascertain with certainty the exact impact on mortality of flu in the UK, because it rather depends on whether a death was directly caused by or as a result of flu, or whether flu indirectly affected a patient, perhaps by aggravating another pre-existing condition, and there are patients who are more prone to succumbing to flu e.g. those with chronic lung conditions or ischaemic heart disease, which is why such patients are advised to have the flu vaccination. In the UK, on average about 600 people die from flu every year, but during epidemics this figure can rise to in excess of 10,000.

Influenza is a tricky virus to prepare for because its H and N proteins change, which means that the vaccine is not perfect. The two primary factors that effect change are called “antigenic shift” and “antigenic drift”. Drift occurs when there are minor changes in the surface protein of the virus from season to season, which means that people may have a degree of immunity but can still contract the virus, albeit that they tend to have a milder condition. This behaviour is responsible for “seasonal” flu. Shift occurs periodically when there is an emergence of a fully-fledged new subtype of the virus to which antibodies have not yet been produced, and the population may, therefore, initially have little or no immunity. This behaviour is responsible for the occasional pandemics, e.g. bird-flu and swine-flu.

It is shift and drift which keep the vaccine companies on their toes because, in making the vaccine, it has to be anticipated what strains of virus are likely to be circulating the following winter. The vaccine is available in two forms. First, an injection into the deltoid muscle in the upper arm. This is not a live vaccine so you cannot catch flu from it. It stimulates the immune system into producing antibodies to the H and N proteins. Secondly, a nasal spray, which contains live attenuated virus to H1N1, H3N2 and two Influenza B strains, and which is used primarily in children. One of the problems with the vaccine is that it takes about 6 months to manufacture it, so to have it available for inoculation in November, December or January means that which strains to protect against must be decided in the preceding Spring/Summer. Unfortunately, it is simply not possible to know with absolute certainty which strains will be around over 6 months later and in some years the guesses are better than in others. For example, only 39.8% of vaccinees (across all ages) had their flu prevented by the vaccine last year and it was not effective at all in the over-65 year olds against Influenza A. In the preceding year, the vaccine was 52.4% all-age effective, 29.1% in the over-65 year old group.

Notwithstanding the seemingly relatively modest success of the vaccine year-on-year, it is critical not to underestimate the Influenza virus: it is a worldwide killer on a regular basis and, even in the UK where there is a reasonable uptake of the vaccination, morbidity and mortality can reach very significant levels at this time of year, so it is a virus to which we should pay some respect.