The MHRA is carrying out a detailed review of reports of an extremely rare blood clotting problem affecting a small number of people who have had the Oxford/AstraZeneca vaccine.
The problem can also happen in people who have not been vaccinated and it's not yet clear why it affects some people.
The COVID-19 vaccine can help stop you getting seriously ill or dying from coronavirus. For people aged 30 or over and those with other health conditions, the benefits of being vaccinated outweigh any risk of clotting problems.
If you are under 30 and have already had the first AstraZeneca dose without any serious side effects, you should still get the second.
For people under 30 without other health conditions, it's currently advised that it's preferable to have another COVID-19 vaccine instead of the Oxford/AstraZeneca vaccine.
Advice if feeling unwell after vaccination
Call 111 immediately if you get any of these symptoms starting from around four days to four weeks after being vaccinated:
a severe headache that is not relieved with painkillers or is getting worse
a headache that feels worse when you lie down or bend over
a headache that's unusual for you and occurs with blurred vision, feeling or being sick, problems speaking, weakness, drowsiness or seizures (fits)
a rash that looks like small bruises or bleeding under the skin
shortness of breath, chest pain, leg swelling or persistent abdominal (tummy) pain