Last week we learned that under-40s in the UK are to be offered an alternative to the Oxford/AstraZeneca Covid-19 vaccine after the jab was linked to very rare blood clots.
The policy of offering an alternative vaccine previously only applied to the under-30s, but the age threshold was raised after the UK's medicines regulator reported new figures on blood clots last week.
The Joint Committee on Vaccination and Immunisation (JCVI) and the Medicines and Healthcare products Regulatory Agency (MHRA) say there are no new safety concerns.
Here, we answer some of the most common questions about the change.
Why has the age range been extended to the under-40s?
First of all, it has been stressed that this is a "precautionary" move and that there are no fresh concerns over the safety of the AstraZeneca jab.
Public Health Wales said: “The JCVI is not advising against using the AstraZeneca vaccine in 30-39-year olds - only that other vaccines are preferred."
Health exerts say it all comes down to finding a balance between the benefits of the jab and the risks from coronavirus.
While the balance of benefit and risk for the AstraZeneca vaccine is very favourable for older people, it is more finely balanced for younger groups, who do not tend to become seriously ill from coronavirus.
The JCVI has said there is an "extremely small risk" of people suffering blood clots after having the AstraZeneca jab.
But it added that the risk of serious illness with Covid-19 also reduces for younger people, as infection rates fall across the country.
Because there are other vaccines available in the UK, healthy younger people will be offered an alternative to AstraZeneca where possible, as long as it does not significantly delay vaccination.
Professor Wei Shen Lim, Covid-19 chairman for JCVI, said: "We have continued to assess the benefit/risk balance of vaccines in light of UK infection rates and the latest information from the MHRA on the extremely rare event of blood clots and low platelet counts following vaccination.
"As Covid-19 rates continue to come under control, we are advising that adults aged 18 to 39 years with no underlying health conditions are offered an alternative to the Oxford/AstraZeneca vaccine, if available, and if it does not cause delays in having the vaccine."
People under 40 with underlying health conditions could still be offered AstraZeneca, because the risk of them becoming seriously ill from coronavirus is much higher than the risks from the jab.
I'm under 40 and awaiting my second dose of the AstraZeneca jab. Should I still have it?
Health experts have said those who have had a first dose of AstraZeneca and not suffered a serious reaction should have a second dose of the same jab, irrespective of their age.
ITV Wales News spoke to Dr Richard Roberts, head of the Vaccine Preventable Disease Programme at Public Health Wales.
He said: "If you have already had the vaccine and not had a reaction, there are no safety concerns about the second dose.
"None of these safety concerns apply to the second dose of AstraZeneca. People can go ahead and have the second dose, with no concerns at all.
"The first dose gives you good protection, but the second dose gives you stronger and longer protection. So it's very important that you have your second dose."
Dr Richard Roberts: 'No safety concerns' around second dose of AstraZeneca
Can I request a different vaccine for my second dose?
Dr Roberts said: "If you were to have a second dose with a different vaccine, you'd have all the complications of that vaccine as well - so it's better to have the second dose of the vaccine that you had first time."
If I am under 40 and yet to receive my first vaccine dose, what will I be offered as an alternative?
Apart from AstraZeneca, the UK is also using two other vaccines - Pfizer/BioNTech and Moderna.
The UK has an in-principle agreement for 60 million doses of the Valneva jab, with an option to acquire a further 130 million doses from 2022-2025.
The country has also ordered 30 million doses of a coronavirus vaccine developed by Johnson & Johnson, which has been shown to be 66% effective in preventing coronavirus infection.
Both the Valneva and Johnson & Johnson jabs will need regulatory approval for use in the UK, once data from later-stage trials become available.
I'm under/over 40 and pregnant - what vaccine will I be given?
The JCVI recently advised that pregnant women should be offered the Covid-19 vaccine at the same time as the rest of the population, based on their age and clinical risk group.
Public Health Wales said: “As there is more experience of the use of the Pfizer BioNTech and Moderna vaccines in pregnancy, those vaccines are therefore the preferred vaccines to offer to pregnant women."
What are the main symptoms that suggest I could be having a reaction to the vaccine?
Many people experience some side-effects from the coronavirus jabs, and this is common.
The MHRA said that, as a precautionary measure, anyone who has a severe headache which is not relieved with painkillers or is getting worse, should seek prompt medical attention at any point from around four days to four weeks after vaccination.
Other side-effects that may need medical attention include a headache that feels worse when a person lies down or bends over, a headache that is unusual and occurs with blurred vision, feeling or being sick, problems speaking, weakness, drowsiness or seizures, a rash that looks like small bruises or bleeding under the skin, and shortness of breath, chest pain, leg swelling or persistent abdominal pain.
How many people in Wales have experienced clots thought to be linked to the AstraZeneca vaccine?
Figures show that the risk of people suffering blood clots after having the jab is extremely small.
Dr Roberts told ITV Wales News: "We've vaccinated in total around 1.9 million people in Wales.
"More than a million of those will have had the AstraZeneca vaccine, and around a quarter of a million will have had the second dose as well.
"So far, we've seen nine cases of this rare clotting disease - which is the number that the MHRA say is expected to be seen, which is around 10 per million vaccine doses."
Should I be worried about having a coronavirus vaccine in general?
Professor Wei Shen Lim, Covid-19 chairman for the JCVI, said: "The Covid-19 vaccines have already saved thousands of lives and the benefit for the majority of the population is clear - if you are offered the vaccine, you should take it."
Dr Richard Roberts: 'All the vaccines work extremely well and are very safe'