The booster vaccination programme is well underway across Wales and the UK, with NHS staff working day and night in the lead up to Christmas to deliver essential doses.
Many people have reported side effects from the booster vaccines, much like the initial two doses.
But why do only some people feel unwell after their jab while others are fine, and does it depend which vaccine you have?
Read more: How long does it take before a booster jab becomes effective?
What are some of the most common side effects?
Side effects from the booster jabs are similar to those from the initial two doses. The most common side effect is a sore arm at the site of infection, while some people report flu-like symptoms after getting their latest dose.
Dr Christopher Johnson is interim head of vaccine preventable disease programme at Public Health Wales.
Dr Johnson told WalesOnline: "The vaccine works by triggering a response in your immune system. It is that response that gives those side effects.
"The most common of those side effects is just a sore arm at the injection site.
"What we know is that the reporting of these symptoms - a sore arm or a bit of a headache or mild fever - for the booster dose is similar to the reporting rate as it was to the first and second dose."
For some people, the symptoms experienced after vaccination are mild. Others could feel under the weather for several days.
Dr Johnson said: "Many people won't experience anything. For a few people, they might just get something mild, it might last a couple of days. But the really important thing is that shouldn't put anyone off getting the vaccine.
"We know that the booster dose in particular is really important in protecting people from Omicron. That recent top up of the immune system is really important with the Omicron virus and it is the best thing you can do to protect yourself from serious illness and death from Covid.
"And you might get a sore arm."
Below are the listed side effects for the Pfizer and Moderna booster vaccines.
Why do some people experience side effects while others do not?
While some people feel tired and under the weather following their booster, others report no side effects at all.
In an informal poll of WalesOnline staff who have received their booster dose, 83% reported a sore arm after at least one of the doses.
But just half of triple-jabbed staff reported feeling ill in any way. Those that did had varied experiences - some felt terrible for a number of days, while others only had a mild headache for a few hours.
Dr Johnson said: "Some people will experience a reaction with their first dose or second dose and not with others.
"It's variable. From our perspective as a vaccine programme, there isn't something that makes you more likely to get side effects than someone else in the general population."
While the two booster vaccines have slightly different reported side effects, Dr Johnson affirms that one is not worse than the other.
He said: "They all have a slightly different side effect profile. I wouldn't say that some were worse than others. You get a different sort of pattern with each of them.
"It is still always the case that the most common side effect you will get is a bit of redness or soreness at the injection site. That is, for all vaccines, the most common side effect."
Are the vaccines safe?
While many people report experiencing some side effects from their Covid-19 vaccination, serious reactions are extremely rare.
Public Health Wales said all types of vaccinations are safe and effective, and encourage people to attend any appointment they are offered.
Dr Johnson said: "The fundamental message is that all the vaccines are safe and effective. And they are the best way to protect yourself and your family at this time.
"It is really important that people prioritise getting their vaccination when they are offered it. We know that having a booster is the best thing you can do."
If you still have not had your first or second vaccination, it is not too late. Unvaccinated people can still take up those two initial doses in order to gain some protection.
Dr Johnson added: "If you're fully vaccinated, having a booster is important to protect from Omicron. If you are unvaccinated, or you've just had one vaccine and you are eligible for another, the best thing you can do to protect yourself from this virus is to get the vaccines that are offered to you.
"If you're offered a vaccination appointment, please join the hundreds of thousands of other people in Wales who are getting their booster vaccine and doing their best to protect themselves.
"Join them, prioritise it and go and get it."
When can I get my booster?
First Minister Mark Drakeford announced on December 13 that every adult in Wales will be offered a booster by the end of the month, if possible.
Those waiting for their appointment have been discouraged from phoning their local health board to ask for an appointment. However, some health boards will be offering walk-in vaccinations.
Here's what every health board in Wales has said about how to get a booster.
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