People are more likely to experience a serious complication from a disease, rather than from the vaccine for that disease.
All vaccines in use in Australia provide benefits that far outweigh any risks.2–4,9–11,14,15
Benefits of vaccines include the prevention of disease, and reduced chances of getting sick, going to hospital or dying.2–4,9–15
Risks of vaccines may include minor and moderate side effects, and more rare serious side effects.
1 in 15 patients with diphtheria die from the disease, whereas serious side effects from the diphtheria vaccine are very rare.3
1 in 4 people chronically infected with hepatitis B will die from cirrhosis of the liver or from liver cancer. Vaccination can prevent chronic infection with hepatitis B.59
In the case of measles infection, the virus erases the memory cells for other infections like the flu and intestinal diseases.60 As a result, people can become susceptible again to catching these diseases despite being previously immunised against them.61
Many vaccines have the added advantage of generating more effective protection against pathogens than through naturally acquiring the infection. For example, a small amount of tetanus toxin can cause life-threatening disease but is not enough to generate enough protective antibodies to prevent disease if exposed again in the future. By contrast, the tetanus vaccine has enough of the inactivated toxoid in it to generate the level of protective antibodies needed to protect against serious illness.
A local reaction at the injection site, such as redness, is one sign the immune system is interacting with the vaccine. This is normal.
The great majority of side effects that follow vaccination are minor and short-lived.
The most common side effects for all vaccine types are ‘local’ reactions at the injection site, such as redness or swelling, which occur within hours and are caused by the vaccine. More general or ‘systemic’ reactions, such as fever or tiredness, can also occur after vaccination, but studies have shown that they are much less common than local reactions.62
Local reactions are signs that the vaccine is interacting with the immune system to generate a protective response. The nature of these reactions varies depending on the type of vaccine given.
For example, if a person develops a fever due to an inactivated vaccine, they almost always do so within 24–48 hours—the time when the immune system is immediately responding to the components of the vaccine. In contrast, the onset of fever caused by a live attenuated vaccine, such as the MMR vaccine, is delayed for 7–12 days. This is due to the time needed for the attenuated virus in the vaccine to multiply enough to produce a protective response from the immune system.63,64
Symptoms such as fever, rashes, irritability and nasal snuffles are common, especially among children. Consequently, it can be difficult to determine how many of these reactions are caused by a vaccine when the ‘background rate’ (how often it occurs anyway) in the same age group is unknown.
In some cases, these kinds of reactions may be caused by the vaccine. But in other situations, the symptoms may be unrelated, occurring by chance at the same time as the vaccination. For this reason, scientists refer to these kinds of symptoms as ‘adverse events following immunisation’ to indicate that the vaccine may not cause events that follow vaccination.
Many adverse events following a vaccine injection are coincidental.
Current vaccines used in Australia have fewer side effects than previous vaccines.
Safety surveillance systems in countries like Australia require health care providers to report adverse events that occur following vaccination, regardless of the cause. The reports are compared with historical trends to identify any changes requiring special investigation and assess whether these adverse events are vaccine-related.
It can be misleading to rely on the reported numbers of adverse events. Several factors must be considered to determine if an event is coincidental or caused by the vaccine. Many adverse events are coincidental.
Researchers in Finland analysed common symptoms in 581 pairs of twins after one twin received the MMR vaccine and the other was given a dummy vaccine made of sterile salty water (a placebo).63
1–6 days after the injection: the number of adverse events in the twin who received the MMR vaccine was almost identical to those in the twin who received placebo.
7–12 days after the injection: the vaccinated group had a measurable increase in symptoms that are known to be associated with receiving the attenuated measles vaccine, such as fever, irritability and rash. On the other hand, when researchers looked at the frequency of coughing or other cold-like symptoms—which are common symptoms at any time, regardless of vaccination—they found no difference between the two groups.
Some of the symptoms known to occur after MMR vaccine were also seen in the group who received the placebo, but at a lower rate.
In summary, this study showed that many common symptoms that occur after a vaccine is given are not caused by the vaccine, but occur by chance at that same time.63
Medical conditions with unknown causes have been incorrectly linked to particular vaccines. The most prominent example over the past 25 years is the claimed link between the MMR vaccine and autism. Children with autism will often display the first clinical signs in their second year of life, which also happens to be the same time that the MMR vaccine is usually given.
The original suggestion that the MMR vaccine might be linked to autism was made in 1998. A research group proposed that the attenuated measles virus in the vaccine infected the intestine. The research group leader claimed this led to inflammation that resulted in lower absorption of nutrients needed for normal brain development, resulting in developmental conditions such as autism.
Many comprehensive studies subsequently ruled out this suggested link by showing conclusively that rates of autism are the same among children who have and have not been vaccinated. Ultimately, the original report was shown to be fraudulent and was retracted by the medical journal that published it.
Similarly, any link between autism and thiomersal, previously used in small quantities as a preservative in vaccines, has been ruled out.65
Febrile seizures
Febrile seizures are a convulsion in a child caused by a rise in their body temperature, usually a fever.
Potentially worrying side effects, such as febrile seizures, have been reported after vaccination. However, such side effects occur much less often with the vaccine than they would if a person caught the disease itself.2,10,12 This is well illustrated in young children by comparing the frequency of adverse events from the MMR vaccine with the frequency of adverse events with measles itself.10
1 in 10 young children develop a fever after receiving influenza vaccine8
9 in 10 children develop a fever after a proven influenza infection66
About 3 in every 10,000 children who receive the MMR vaccine develop a fever high enough to cause short-lived seizures. In contrast, the risk of such a fever is more than 30 times greater among children who develop the disease—affecting about 100 in 10,000 children. Importantly, measles vaccination has prevented an estimated 23.3 million deaths worldwide.
The frequency of side effects associated with some earlier vaccine preparations (no longer in use in developed countries such as Australia) was higher than with the current generation of vaccines.
Below: Likelihood of severe complications (among 1 million children aged under 5 years) from receiving the MMR vaccine compared to those from having measles2,10–12
|
|
COMPLICATION |
MMR VACCINE |
MEASLES |
|---|---|---|---|
Uncommon complications |
Seizures or convulsions brought on by fever |
300 children have seizures |
10,000 children have seizures or convulsions induced by fever |
Rare complications |
A temporary tendency to bruise or bleed more easily (thrombocytopenia) |
26 children develop thrombocytopenia |
330 children develop thrombocytopenia |
Very rare complications |
Severe allergic reactions (anaphylaxis) |
Up to four children have a severe allergic reaction. This is readily treated with complete recovery |
No anaphylaxis cases |
|
|
Inflammation of the brain (encephalitis), which may result in permanent brain damage or death |
A maximum of one child may develop encephalitis |
2000 children may develop encephalitis |
|
|
Subacute sclerosing panencephalitis (SSPE), which causes progressive brain damage and death |
No children will get SSPE |
10 children get SSPE several years later |
Autoimmune disease
Autoimmune diseases occur when the immune system mistakenly attacks the body instead of invading pathogens.
Over the past 30 years, the number of people who develop autoimmune diseases has been increasing, particularly in societies where rates of infectious disease have declined. This has raised the question of whether vaccine use can lead to the development of autoimmune disorders. Except for the two rare diseases mentioned below, the answer is no. This conclusion is based on the stringent monitoring procedures put in place for detecting side effects of vaccination.
The first exception is the small increase in the risk of developing the rare condition known as immune thrombocytopenic purpura, a condition where blood fails to clot normally, after receiving the MMR vaccine. However, the risk of developing this disorder associated with measles infection itself is more than 10 times greater than that associated with the vaccine.
The other exception is Guillain–Barré syndrome, a nerve condition, following influenza vaccination. Again, the risk of developing the disease after the influenza vaccination is much lower than after influenza infection.
Like autoimmune diseases, allergic diseases such as asthma have become more common in the developed world over the past 30 years. However, there is no significant evidence that vaccines cause allergic diseases in otherwise healthy people.
Generally, for every 100,000 doses of a vaccine, less than 1 person will experience a severe allergic reaction after receiving it—a rate that is extremely low.43 Nevertheless, people with a history of reactions to a specific vaccine or vaccine additives, or a strong family history of allergic disease, should always take precautions.
Injectable vaccines used in Australia do not contain detectable amounts of antibiotics such as penicillin or sulphonamides to which some people may be allergic. The hepatitis B vaccine is grown in yeast. Although there have been some isolated reports of possible severe allergic reactions to this vaccine, the benefits of receiving the vaccine far outweigh the multiple risks associated with hepatitis B infection.
Careful testing of vaccine safety is an essential part of vaccine development and manufacture. There are also ongoing surveillance programs after vaccines have been introduced into the community.
Before a vaccine can be developed, research is undertaken to better understand the pathogen and the disease it causes, helping to determine how potential vaccines are likely to work.
During vaccine development, safety testing procedures occur in multiple stages:
The first stage involves preclinical assessment in the laboratory, usually using animals.67 If a vaccine fails these safety tests, it cannot progress into clinical trials.
Vaccines are then evaluated in three phases of clinical trials:
In Phase I clinical trials, the potential vaccine (or ‘vaccine candidate’) is given to small numbers (typically 25–50) of healthy adults with the primary goal of assessing safety.
Phase II clinical trials involve hundreds of participants and are designed to show how good a vaccine is in provoking an immune response and determining the best dosage to use.
Phase III clinical trials aim to demonstrate a vaccine’s safety and how well it protects against the target disease across different groups of people, which usually requires administering the vaccine to many thousands of potentially susceptible people. Only after the vaccine has passed each of these safety and efficacy hurdles is it approved for widespread community use.
Some side effects of vaccines are so rare that they are not detected during the extensive safety testing before a vaccine is approved for use. To ensure that even very rare side effects are detected, careful surveillance continues even after a vaccine candidate has proven to be effective and has passed all safety checks. The formal term for this collection of information and reporting of any suspected adverse events is post-licensure assessment.
If a potential problem with a vaccine is detected, the use of that vaccine may be temporarily paused. This allows health authorities to investigate what’s caused the problem and whether it can be linked to a particular batch of the vaccine. For example, young children given a particular influenza vaccine in Australia in 2010 showed an increased risk of febrile seizures. When the problem first became apparent, the use of all influenza vaccines in young children was suspended to allow authorities to identify the one type of vaccine preparation causing the problem. Then, influenza vaccines shown not to be associated with unacceptable rates of febrile seizures were reintroduced to ensure that protection against influenza remained available for children at high risk of complications from the disease.
Vaccine safety and effectiveness: how is it tested?
© 2024 Australian Academy of Science
