Skip to main content

Worried about having your second dose of AstraZeneca?

Like all medicines, vaccines can cause side effects. Most of these are mild and short-term, and not everyone gets them.

It is important that you get your 2 doses of the COVID-19 vaccines to give you the best protection.  Not all COVID-19 vaccines are the same.

Very common side effects in the first day or two include:

  • having a painful, heavy feeling and tenderness in  the arm where you had your injection
  • feeling tired
  • a headache
  • general aches, chills or flu-like symptoms

The AstraZeneca (AZ) vaccine causes fewer side effects after the second dose.

The Pfizer and Moderna COVID-19 vaccines tend to cause more side effects after the second dose. Studies have shown that if you have one of these vaccines after a first dose of AstraZeneca, your risk of common side effects is higher.

The current advice from the Joint Committee on Vaccination and Immunisation (JCVI) is:

Those who have received their first dose of AZ vaccine without suffering any serious side effects should continue to have the second dose to complete the course. This includes individuals who are aged 39 years or younger.

Rare side effects

Since March 2021 there have been reports from the UK and internationally of a very rare condition of thrombosis (blood clots) and thrombocytopenia (low platelets). In under 50s, around 1 case has been reported for every 50,000 first doses of the AZ vaccine. For the second dose of vaccine, less than 1 case has been reported for every million vaccines given to people under 50 years of age, none of these second-dose cases were confirmed.

The underlying risk factors have not yet been fully established for this condition and a detailed review of suspected cases is ongoing by the Medicines and Healthcare products Regulatory Agency (MHRA), supported by public health bodies and other professional groups. The advice from these expert groups is that unless you had this rare clotting event you are best to complete with this vaccine for your second dose.

It is recommended that you have both doses of the same vaccine. In exceptional circumstances where this is not possible it is better to have a second dose  of another vaccine than not at all.

Reporting suspected cases of blood clots and low platelets, and other side effects

It is very important that all suspected cases are reported to the MHRA on the COVID-19 Yellow Card scheme.

A blood clotting factsheet for patients receiving their first dose is available to read here:

Further information and patient leaflets can be found at: