Skip to 0 minutes and 14 seconds In this short video, we’re going to look at the epidemiology of influenza. Epidemiology is the science of epidemics, how diseases spread, how fast and by what routes. Pandemic and seasonal influenza are the same disease. But pandemics are rare and they involve novel subtypes whereas seasonal flus involve the same subtypes coming back year after year. Pandemics are generally much more severe, although bad years for seasonal flu can be almost as bad.
Skip to 0 minutes and 43 seconds But despite these differences, the virus is spread the same way, by respiratory secretions, that’s the coughs and sneezes aerosoling into crowded rooms for inhalation by the next victim, or on the other hand, by those secretions landing on surfaces and lying in wait for a hand to touch them, that will then go into a mouth or nose or an eye. And that’s what we call fomite transmission. Part of the reason we do epidemiology is that if we can understand how a disease spreads, we can then think of ways to prevent it from spreading. Viruses in general are difficult to treat once you’ve caught them.
Skip to 1 minute and 20 seconds Their simple structures and the fact that they hijack normal human cellular processes to replicate themselves means that it’s very difficult to design a drug that will attack a virus without harming the host. And as a result, prevention has tended to take precedence over cure. For flu, the simplest method of prevention is to try to tackle aerosol and fomite transmission. Some of you doing this course may live in East Asia. And in some countries in this region, it’s normal to see people wearing face masks. Usually, these are worn by people suffering from respiratory viruses to catch the coughs and sneezes and prevent them reaching others.
Skip to 2 minutes and 0 seconds In times of flu pandemic, however, people might start wearing them as a preventative measure to stop viruses going in. In the West, the handkerchief is the preferred measure. It’s more comfortable, but also tends to be less efficient. If you’re not quick enough, a sneeze can get out before the handkerchief reaches your face. Handkerchiefs also tend to result in the user getting virus on his or her hands, which can then be passed on by touching others, shaking hands, et cetera. That’s fomite transmission again. So this brings us to our second line of defence, which is hand hygiene.
Skip to 2 minutes and 34 seconds If the use of handkerchiefs or just simply sneezing into one’s hand results in the potential source of fomite transmission, then the obvious way to counteract that is to make sure that the hands are cleaned at regular intervals. And this doesn’t just apply to sufferers. Washing the hands before eating might remove any virus particles that you’ve acquired touching door handles, tapping keyboards, shaking hands, or using a shared office telephone, for instance. But of course, there are numerous technical issues surrounding these kind of measures. How effective, for instance, is a face mask? What kind of filter does it need? How thoroughly do we need to wash our hands? Does the kind of soap or detergent we use matter?
Skip to 3 minutes and 17 seconds What about hand gel products, for instance, for hand cleaning without water? And what about contaminated hand towels? And do hand dryers make things worse or better? These details are crucial and remain an active area of debate. In hospitals, the tendency is to err on the side of caution. Because so many patients in hospitals are already very ill, the last thing that’s needed is a flu outbreak on the wards. And flu isn’t the only pathogen of concern. Hospitals suffer particularly badly from norovirus, which causes diarrhoea and vomiting, and also from drug resistant strains of common bacteria. To help minimise these threats, an especially thorough hand washing method, called the Ayliffe technique, is now used.
Skip to 4 minutes and 1 second However, the Ayliffe technique is probably too cumbersome for everyday use, although perhaps in a major flu pandemic, people would start to think seriously about using it. Altering our behaviour and increasing our hygiene may be one method to keep flu transmission down. But equally important is vaccination. The principal is simple. The adaptive immune system will produce antibodies against infection which will then protect to varying extents, depending on the circumstances, against subsequent infections with the same pathogen. In mediaeval times in China and Africa, an early variant of vaccination called variolation was used. Material from pustules in mild cases of smallpox were used to infect other people with what it was hoped would also be a mild case.
Skip to 4 minutes and 49 seconds Now this was quite a gamble, as you might imagine. The patient had to be willing to risk the possibility that full blown smallpox transmission would occur. Variolation spread to Europe in the 18th century, where the next advance occurred. Edward Jenner observed that dairy farm workers rarely suffered from smallpox but often caught the much milder cow pox from their animals. Instead of variolating with smallpox pustule derived material, Jenner used cow pox pustules for the same process, resulting in a much safer option, now named vaccination in reference to the Latin word for cow. Around 100 years after Jenner, Louis Pasteur was instrumental in moving vaccination into the medical mainstream, producing the first vaccines against cholera, anthrax and rabies.
Skip to 5 minutes and 36 seconds Fairly early on in the development of vaccination, it was realised that the pathogen doesn’t need to be alive in order to elicit an immune reaction. The immune system will produce antibodies against non-functional bits of virus just as well as against an active virus particle. The use of vaccines containing live virus without the consequent risks that an actual infection would be caused was ended. Some vaccines are therefore designated inactivated, meaning the virus is effectively dead, and others are attenuated, meaning that the virus is still alive but not capable of causing the disease. Both inactivated and attenuated vaccines against flu are used. Vaccination can therefore reduce the impact of influenza, just as it was done for numerous other infectious diseases.
Skip to 6 minutes and 25 seconds But we are unlikely ever to see anything similar in the flu world like the eradication of smallpox that we saw in the 1970s. The reason for this is that influenza viruses evolve so quickly. Unlike the smallpox virus, which evolves very slowly by comparison, flu is changing its surface protein structure every year. This is the process of antigenic drift. So a vaccine against this year’s flu will gradually become more and more useless as the years go by. To remain as protected as possible against influenza, you need to receive the vaccine every year. I say as protected as possible, rather than completely protected, because antigenic drift can sometimes go in unexpected directions.
Skip to 7 minutes and 5 seconds We might say that the vaccine designers are outwitted by the virus. But of course, influenza has no wits or brain of any kind. It’s all just a process of random change and natural selection, evolution, in other words, which is sometimes reasonably predictable and sometimes not. Vaccine design for the following year’s seasonal flu vaccine begins in the midst of this year’s flu season. Scientists study the characteristics of strains circulating in the present season and then compare them to previous seasons. They look to see if there are any irregularities, and they use sophisticated computer software to analyse those irregularities and formulate predictions.
Skip to 7 minutes and 44 seconds At the end of this process, they’ll have an idea of which currently circulating strain is likely to be most similar to whatever arrives next year. Vaccine production then needs to start immediately, so that stockpiles are ready for the following year. And stockpiles really are what are needed. The annual vaccination task is immense. Since millions of people all over the world need to be vaccinated, and since public health systems funded by taxpayers cannot really either afford to vaccinate everybody, or have the time and the personnel to do so, a certain amount of prioritisation has to be carried out.
Skip to 8 minutes and 19 seconds The elderly and those with pre-existing medical conditions that might make an attack of flu very dangerous are at the front of the queue. However, if you have enough money of your own, about 10 pounds in the UK, that’s around $16 US, you can pay for your own vaccination at a commercial provider, such as a pharmacy. As well as taking some of the financial burden off the public purse, getting vaccinated helps other people, as well as you, since you’re then less likely to pass the flu on to others. There’s one other preventative measure, which is the most low-tech of all, simply avoiding other people when you’re ill.
Skip to 8 minutes and 55 seconds If nobody’s within range, nobody can be on the receiving end of your aerosols and nobody will touch any virus that you deposit on a fomite. This is referred to as social distancing. Stay off work and find something else to do at home, like for instance, another excellent course from Future Learn.
Epidemiology is the study of epidemics. You might think that there also ought to be a “pandemiology” to study pandemics, but all kinds of infectious disease, whether local epidemics, global pandemics or persistent endemics, are covered under the umbrella of epidemiology.
The fundamental questions in epidemiology are:
- How do diseases spread?
- How fast is this spread?
- Who catches them?
- How can we intervene to prevent spread?
In this step, we’ll be considering the way that flu is spread from person to person, via aerosols and fomites, and then look at ways we can prevent such transmission. Then we’ll go on to consider vaccination, from its origins in variolation, through the pioneering work of Jenner and Pasteur to today’s annual centrally co-ordinated global flu vaccination efforts.
As you watch the video, think about how the immune system (which we discussed last week) defends the body from a virus infection. Then think about how vaccination performs a sort of “dry run” or “rehearsal” for the immune system.
Some vaccination campaigns of the past have been enormously successful - such as the campaign to eradicate smallpox, the last active case of which occurred in Africa in the late 1970s. Other diseases have had their incidence reduced dramatically in recent years due to vaccination, but have not quite achieved eradication.
Two prominent examples in this category are measles and polio. Measles vaccination has unfortunately fallen foul of anti-vaccination campaigners, resulting in public rejection of the campaign in some countries. Polio eradication is a distinct prospect, but conflicts and wars around the world mean that some of the people who need polio vaccination the most, cannot be accessed by vaccination teams.
Measles and polio eradication are achievable, but unfortunately are unlikely to occur soon because of these largely political reasons. Influenza eradication, however, is probably not a realistic prospect at all, for reasons explained in the video. If you’ve already watched the video on antigenic drift, you’ll probably already be able to say why.
Links to some further reading can be found below.
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