The novelty of the COVID-19 vaccines may seem daunting for some, and it is natural for questions to arise on their effectiveness. In this feature, we examine the difference between effectiveness and efficacy, compare the COVID-19 frontrunner vaccines to other vaccines, such as the flu shot, and compare their safety considerations.
As Pfizer/BioNTech roll out their COVID-19 vaccine throughout the United Kingdom and the United States, the world wonders how effective it will be.
Looking at the three leading vaccines that we have previously reported on, Pfizer/BioNTech boasts 95% efficacy, the Oxford/AstraZeneca vaccine candidate has an average of 70% efficacy, while the Moderna vaccine candidate reportedly has 94.1% efficacy.
But what does this say about their effectiveness? And how does it compare with vaccines against the flu, polio, and measles?
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Effectiveness vs. efficacy — what is the difference?
Firstly, it is worth noting that “effectiveness” and “efficacy” are not the same. Despite news outlets frequently using them interchangeably, efficacy refers to how a vaccine performs under ideal lab conditions, such as those in a clinical trial. In contrast, effectiveness refers to how it performs in the real world.
In other words, in a clinical trial, a 90% efficacy means that there are 90% fewer cases of disease in the group receiving the vaccine compared with the placebo group.
However, the participants chosen for a clinical trial tend to be healthier and younger than those in the general population, and they generally have no underlying conditions. Furthermore, researchers do not normally include certain groups in these studies, such as children or pregnant people.
So, while a vaccine can prevent disease in a trial, we might see this effectiveness drop when administered to the wider population.
However, that is not in itself a bad thing.
Flu shot effectiveness
Vaccines do not need to have high effectiveness to save thousands of lives and prevent millions of disease cases.
The popular flu shot, for example, has an effectiveness of 40–60%, according to the Centers for Disease Control and Prevention (CDC).
However, during 2018–2019, it prevented around “4.4 million influenza illnesses, 2.3 million influenza-associated medical visits, 58,000 influenza-associated hospitalizations, and 3,500 influenza-associated deaths.”
It is also worth noting that the flu vaccine’s effectiveness varies from season to season, due to the nature of the flu viruses circulating that year. Determining the precise rate of effectiveness can be challenging.
Finally, it bears mentioning that the number of doses can also improve effectiveness for some vaccines. For the flu shot, two doses of the vaccine instead of one can offer a protection boost, but this benefit is limited to only a few specific groups, such as children or organ transplant recipients.
The booster dose does not seem to benefit people over the age of 65 or those with a compromised immune system.
By contrast, as we will see below, for vaccines, such as the ones against polio and measles, a higher number of doses is required to achieve peak effectiveness.
Polio vaccine effectiveness
The vaccine against polio can be up to 100% effective. According to the CDC, “Two doses of inactivated polio vaccine (IPV) are 90% effective or more against polio; three doses are 99% to 100% effective.”
However, experts recommend IPV in four doses. The CDC recommend that children take one dose of the vaccine at 2 months old, another dose at 4 months old, a third dose at 6–18 months old, and a final fourth dose at 4–6 years old.
These four doses combined yield 99–100% effectiveness.
The IPV vaccine protects against poliomyelitis, a disease caused by the poliovirus, which can trigger an infection in a person’s brain and spinal cord, causing paralysis.
Measles vaccine effectiveness
The MMR vaccine, which protects against measles, mumps, and rubella, can be up to 97% effective at preventing measles when administered in two doses.
One dose is approximately 93% effective, according to the CDC.
The CDC recommend administering the first dose at “12 through 15 months of age, and the second dose at 4 through 6 years of age.”
How do the side effects compare?
When it comes to new vaccines, the issue of safety is understandably on everyone’s minds.
The question “how safe is this vaccine?” can be translated as “what is the risk of developing side effects from this vaccine?” And, in answering this query, it is important to note there is no drug — not even the most common painkiller — that is entirely free from side effects.
Scientists have broken down the side effects that they expect to occur with the BioNTech vaccine — currently the only one with approval for use in the U.K. and the U.S. — into “very common,” “common,” and “uncommon.”
Very common side effects, which may affect more than 1 in 10 people, include:
- pain at the injection site
- tiredness
- headache
- muscle pain
- chills
- joint pain
- fever
The common side effects, which may affect up to 1 in 10 people, are:
- injection site swelling
- redness at the injection site
- nausea
Uncommon side effects, which may affect up to 1 in 100 people, include enlarged lymph nodes or feeling unwell.
By comparison, the CDC list the following side effects for the flu shot as common:
- soreness and redness, swelling, or both, from the shot
- headache
- fever
- nausea
- muscle aches
Occasionally, they add, the flu shot may cause fainting, like any other injections. The CDC recommend that everyone take the flu vaccine, except for children younger than 6 months of age or those with severe allergies to the flu vaccine or any ingredient in the vaccine.
Some studies have found a slight risk of Guillain-Barré syndrome after flu vaccination, while other studies found no association. For those that did find such a likelihood, this risk was 1–2 in 1,000,000 people.
For MMR, the side effects listed as common by the CDC are:
- sore arm from the shot
- fever
- mild rash
- temporary pain and stiffness in the joints
Rarely, the vaccine may trigger febrile seizures, swelling in the cheeks or neck, or a temporary low platelet count in the blood. However, none of these are life threatening or have long-term effects.
For polio, the CDC note that “A sore spot with redness, swelling, or pain where the shot is given can happen after a polio vaccine.” In addition, “As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.”
COVID-19 vaccine and allergic reactions
Since rolling out the BioNTech vaccine in the U.K., two recipients — who had a history of serious allergies — developed allergic reactions.
This prompted regulators to recommend people who have a history of significant allergic reactions do not have the COVID-19 vaccine.
While erring on the side of caution is preferable in the early stages of rolling out a vaccine, it is worth noting that all vaccines — including the ones for polio, flu, and measles — have a risk of triggering severe allergic reactions.
This risk stands at around 1 in 900,000 people.
With the COVID-19 vaccine and vaccine candidates, the safety data we have so far come from tens of thousands of people receiving the vaccine in the Pfizer/BioNTech, Moderna, and Oxford/AstraZeneca trials.
So, the safety information we can obtain based on this number of participants is enough for regulators to decide whether a vaccine is safe enough to license. However, it may not be enough to detect rare events that could affect 1 in 100,000 people, for example.
As Dr. Charlie Weller, Head of Vaccines at the Wellcome Trust in London, U.K., points out, “Much more data would be needed to determine if there is any relationship between the vaccine and these [allergic] reactions. Guidance would then be issued to those who might be at risk.”
“As vaccine rollout is just beginning, many unanswered questions remain. Ongoing monitoring will help us identify any consistent patterns of adverse events. As is normal for any vaccine, close and continued monitoring for safety and efficacy data as it is delivered will be essential.”
– Dr. Charlie Weller
“For the general population, this does not mean that they would need to be anxious about receiving the vaccination,” adds Prof. Stephen Evans, Professor of Pharmacoepidemiology at the London School of Hygiene & Tropical Medicine.
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