How do vaccines work?

It is estimated that vaccinations against common diseases now save 57 million lives every year worldwide. This is almost the same as saving the entire population of Italy, annually. Humans have been living with diseases throughout history. As far back as 1200 BC there is evidence of influenza being described in Babylon. Since then, humans have been documenting disease outbreaks around the world, with the latest being COVID-19.

Dr Jenner and smallpox

In the 1700s, smallpox was wide-spread with high death rates. During this time, a doctor in England, Edward Jenner, noticed that cowpox, an illness in cattle and usually mild, could be spread from a cow to humans via sores on the cow. During an infection, dairy workers get sores on their hands and spread the infection to other parts of their body. The cowpox virus belongs to the family of viruses that also includes smallpox.

Jenner tested the idea that an infection with cowpox could protect a person from smallpox. On May 14, 1796, Jenner inoculated an 8-year-old boy with some matter taken from a cowpox sore on the hand of milkmaid. The boy suffered a local reaction and felt poorly for several days, but made a full recovery. Two months later, Jenner deliberately infected the same boy with matter taken from a fresh human smallpox sore, in an attempt to show the protection from cowpox. The 8-year-old boy did not develop smallpox.

Jenner next demonstrated that cowpox matter transferred in a human chain, from one person to the next, provided protection from smallpox. Initially, Jenner was ignored, but this changed when a colleague of Jenner’s demonstrated that vaccination with cowpox material prevented future smallpox infections. From that point on, the word spread quickly about how to protect people from smallpox.

Mass vaccination

The first mass vaccination programs began in the early 1900s. Today, in Australia we have the National Immunisation Program (NIP). This program is aimed at giving all Australians an opportunity to be vaccinated against the most common diseases in Australia. The program literally begins from birth, with a vaccination against hepatitis B. By school age, most children are now immunised against diphtheria, tetanus, whooping cough (pertussis), hepatitis B, polio, haemophilus influenzae type B (Hib), rotavirus, pneumococcal, measles, mumps, rubella, meningococcal C and chickenpox. In high school, children are also immunised against human papilloma virus (HPV), a leading cause of cervical cancer.

During their life, people will need booster shots of some vaccines every 10 years to maintain their immunity. This includes diphtheria, tetanus and whooping cough.

Some of us will also have a yearly flu shot to keep us protected against the ever-changing influenza virus. For those 70-80 years of age, there are vaccines against shingles. This is for people who were never vaccinated against chickenpox as to develop shingles, you must have first been infected with chickenpox.

Australian travellers may have some other vaccinations depending on where they plan to travel to. Common travel vaccines are hepatitis A, typhoid, cholera, Japanese encephalitis and tuberculosis. Farmers are encouraged to vaccinate against Q fever.

Children and adults may receive different types of vaccines and different doses depending on their age.

How do vaccines work?

Vaccines work because they contain weakened or inactive parts of a particular bacteria or virus (antigen) that trigger the body’s immune system to respond. Newer vaccines contain the blueprint for producing antigens, rather than the antigen itself. Regardless of whether the vaccine is made up of the antigen or the blueprint, this weakened version will not cause the disease in the person receiving the vaccine. However, it will prompt their immune system to respond much as it would on its first contact with the actual disease.

This means that some people will feel symptoms similar to those as if they had contracted the real disease. Commonly people report a headache, mild fever or temperature, general aches and pains and feeling a bit unwell. Most of us who have had a Tetanus injection will be familiar with the feeling of a ‘dead arm’ after having this injection. These side effects are common to lots of immunisations.

Severe side effects, or an anaphylactic reaction, are rare. When the side effects are more than just a mild reaction, but not severe, some people think they have developed the actual disease. We hear people claim they got the flu from the flu shot. This is not correct. What they experienced was their body’s immune system responding to the vaccination. This means that if they do come into contact with the real virus their body will recognise the it and know how to fight it off.

Diabetes and vaccinations

People living with diabetes may also experience other signs or after-effects of immunisations, in addition to those described above. Most commonly, you may notice a rise in your blood glucose levels (BGLs).

When people living with diabetes become sick, particularly when severe, this can lead to hyperglycaemia or high BGLs due to the body releasing hormones such as cortisol and adrenaline. This then causes an increase in glucose production by the liver, which leads to a reduction of glucose uptake by the body, causing increased insulin resistance.

This response by the body to illness is very similar to what happens when we have a vaccination. This means some people will notice their BGLs rise following vaccinations.

If this happens, do not be alarmed. It is a fairly common side-effect of vaccinations in people living with diabetes. Some people will not notice any effects, but others may see a significant rise in BGLs. In some people it may mean you need to temporarily increase some of your diabetes medications to manage your diabetes.

Other people may see only a small rise in BGLs, so may be able to accept a short-term rise in their BGLs. Usually, the effect on BGLs is only for a few days, so it will have minimal impact on your overall diabetes management.

Do some vaccines have a greater impact on BGLs than others?

There are a few vaccines that seem to have a greater impact on BGLs than others. In some years the flu vaccine appears to affect BGLs more than other years. Clinicians are noticing that the COVID-19 vaccines cause BGLs to rise in a significant number of people following their first dose. Clinicians are also receiving reports that indicate the BGL rise can last from a few days up to two weeks following the vaccination.

This should not be a deterrent to having your COVID-19 vaccination, as with some planning you will be able to manage any BGL rise. You do not need to be alarmed. If you notice a significant rise in your BGLs, follow your ‘Diabetes Sick Day Management Plan’ or contact your diabetes educator or doctor for advice.


Final words

The bottom line is the risk of some side-effects, either mild or moderate, outweigh the possibility of being very ill or dying from a disease. History shows us the huge benefits of immunisations in saving people’s lives. Polio has nearly disappeared, with a 99% reduction in cases worldwide since 1988. In 1980, the World Health Organization declared the world was finally free from smallpox, nearly 200 years after Edward Jenner discovered how to protect people. These results were only achieved because of vaccinations.


Malcolm Flood, Registered Nurse, Credentialled Diabetes Educator and Authorised Nurse Immuniser.

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