Are Live Vaccines Safe?


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There are a number of different types of vaccination employed by health services today, and each of these is tailored to effectively immunise against the wide variety of bacteria, viruses, and other pathogens we encounter every day. One particular class of vaccines, live attenuated vaccines, are particularly widely used because of how effectively they can offer immunity against even the most virulent and potent of diseases.

Examples of live vaccines include the MMR vaccination and polio vaccine, both of which are invaluable methods of preventing the onset and spread of serious conditions. While the use of live vaccines is now widespread, there are sometimes safety concerns which this article will address.

Why are there safety concerns regarding the use of live vaccines?

Live attenuated vaccines have been instrumental in routine immunisation programmes in the UK, and while their use is widespread and has been for many years, some people raise valid concerns about this particular type of vaccine.

These concerns stem from the nature of this particular class of vaccine, which makes use of variants of the actual micro-organisms responsible for disease. A live vaccine is developed by carefully growing versions of disease causing bacteria and viruses in such a way that they are attenuated or weakened, so that once exposed to the human body they don’t pose a serious threat to the immune system. Despite this attenuation, these vaccines are excellent methods of getting the body to respond to the virus or bacteria in question because they are as close to the actual infection that we can safely get. Which means that they are particularly effective at achieving their immunisation goals.

It is not unreasonable to worry about being exposed to a live virus or bacterium, but you should rest assured that the strains developed for vaccination are carefully and routinely tested to ensure their safety.

Another concern in this area is about a phenomenon called ‘reversion’, which is where an attenuated vaccine sometimes undergoes spontaneous genetic changes which make it more virulent and like the ‘wild’ species. While this is, again, a valid concern, steps are taken to alter the genetic material of viruses and bacteria so that they can’t revert.

All medical treatments, including vaccination, carry with them a small risk of inducing an allergic reaction. While these are usually extremely mild, in extremely rare cases a person can undergo an anaphylactic reaction to the vaccine which can be quite severe. Fortunately medical staff are specifically trained to be able to deal with this kind of event and can rapidly administer treatment.

So are live vaccines safe?

In short, yes. This type of vaccination has been rigorously tested, and a number of live vaccines have been used across the world for decades with little to no adverse effects beyond the standard side effects a vaccine can cause. These include some mild symptoms like a fever or inflammation around the injection site, and these are just an indication that the vaccine is working, inducing your body’s immune response against the live virus or bacteria contained therein. Of course if more severe symptoms present you should always consult your doctor.

Live vaccines are not given to pregnant women or people who are immunocompromised for safety reasons. This has nothing to do with the safety of the vaccine as such, and is instead a consequence of the fact that people in these categories are more vulnerable.

Pregnant women can potentially pass on the components of a live vaccine to their unborn baby, and this can have particularly detrimental consequences. People who are immunocompromised, whether because of certain cancer treatments or because of an infection like HIV which weakens the immune system, can’t safely take the vaccine because their innate defences wouldn’t be able to effectively cope with the attenuated agents involved.

Beyond these exceptions you can rest easy that live vaccines are in fact a very safe method of immunising people of all ages against dangerous conditions.


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