Imagine a seatbelt.
A person wearing a seatbelt can still be injured in a car accident.
That doesn’t mean the seatbelt provides no protection.
The same basic principle applies here, although vaccines and seatbelts obviously work in very different ways.
A vaccine does not promise that nobody will ever become infected.
Instead, vaccination changes the probability of different outcomes.
And those probabilities can change depending on age, underlying health conditions, previous infection, circulating variants, time since vaccination and other factors.
Why Can Vaccinated People Still Get COVID?
There isn’t one single explanation.
Several factors can contribute.
1. Immunity changes over time
Protection is not necessarily permanent.
The WHO reports that protection against severe disease and death is highest in the first few months following vaccination and subsequently decreases.
This does not mean protection suddenly disappears.
It means the level of protection can gradually decline.
That is why vaccination recommendations can change as scientists collect additional evidence.
2. The virus changes
SARS-CoV-2 has produced multiple variants over the course of the pandemic.
Some variants have been able to spread more efficiently or partially evade existing immunity.
The WHO notes that viruses can change over time and that emerging variants can affect vaccine protection against infection.
This is one reason updated vaccines have been developed.
3. No vaccine is 100% effective
This is not unique to COVID-19.
The WHO emphasizes that no COVID-19 vaccine provides 100% protection against infection.
So a person can follow vaccination recommendations and still become infected.
The important question is what happens after infection.
4. People respond differently
Two people can receive the same vaccine and develop different levels of immune protection.
Age and health can influence immune responses.
People with weakened immune systems may not respond as strongly as healthy people.
The WHO notes that people at higher risk of severe disease include older adults and people with certain underlying conditions or compromised immune systems.
Does Vaccination Cause COVID-19?
This is another point that often becomes confused online.
COVID-19 vaccines do not cause a person to become infected with SARS-CoV-2 simply because they received the vaccine.
The WHO explains that approved COVID-19 vaccines do not contain the live virus that causes COVID-19 and therefore cannot make someone sick with COVID-19.
