What is known about the AstraZeneca COVID-19 vaccine (in use overseas) and blood clots?

A very rare clotting disorder has been reported overseas following vaccination with viral vector COVID-19 vaccines in younger adults.

A very rare and new type of adverse event has been observed following vaccination with the AstraZeneca, and more recently the Janssen, viral vector COVID-19 vaccines overseas (including Australia). Although, the reason why this may occur is unknown, it has been proposed the vaccine can induce an immune response where the body incorrectly attacks its own cells to make platelets very sticky. This results in the formation of blood clots in unusual places such as the brain or abdomen. This is similar to a recognised side effect of heparin, a commonly used medication used in hospital.

The unusual characteristic of this type of clotting disorder (thrombosis) is that it is also associated with bleeding due to a lack of platelets in the blood (thrombocytopenia) - the sticky platelets clump together to form clots and this attracts more platelets from the blood - named thrombosis with thrombocytopenia syndrome (TTS). The only risk factor that has been identified so far is age - most cases have occurred in adults aged under 50 years. See the Brighton collaboration for the interim case definition.

This is a very rare event (around 7-10 cases per million doses in those aged under 50 years) and the risk from COVID-19 is significantly higher than the risk of TTS. For this reason, regulatory authorities overseas are weighing up the benefit and potential risk of this vaccine and some have advised age related precautions.

A MedSafe review, published 27 April, finds there is no current evidence of risk associated with the Comirnaty (Pfizer/BioNTech) vaccine and blood clots.

The AstraZeneca COVID-19 vaccine is not yet licenced for use in New Zealand.

To find out more about how vaccine safety is monitored and the risk of blood clots from COVID-19, COVID-19 vaccines and other causes, see this commentary.

References:
https://www.ema.europa.eu/en/news/astrazenecas-covid-19-vaccine-ema-fin…
https://www.gov.uk/government/publications/covid-19-vaccination-and-blo…

https://www.who.int/news/item/16-04-2021-global-advisory-committee-on-v…-(vaxzevria-and-covishield)

Why has the Janssen COVID-19 vaccine roll out been paused in some countries internationally?

Out of an abundance of caution, the roll out of the Janssen COVID-19 vaccine has been paused while further information on rare unusual clotting abnormalities can be collected.

As of 12 April 2021, six cases of abnormal clotting were reported to the Vaccine Adverse Event Reporting System (VAERS) in the US. While this is only a small number compared to the ~6.8 million doses administered, it is important all information is appropriately reviewed and this pause gives scientists the opportunity to do so. In the New Zealand setting, the only vaccine which has been approved for use is the Pfizer-BioNTech COVID-19 vaccine. There have been no reports of this particular clotting concern following 97.9 million doses administered of the Pfizer-BioNTech COVID-19 vaccine. Medsafe will carefully evaluate all the available and emerging data before approval to use can be given.

A MedSafe review, published 27 April, finds there is no current evidence of risk associated with the Comirnaty (Pfizer/BioNTech) vaccine and blood clots.

Reference: 
https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2021-04/03-…

How many doses of COVID-19 vaccine should I have? I already received a COVID-19 vaccination overseas.

If you were partially vaccinated overseas with one dose of Comirnaty (Pfizer/BioNTech) vaccine, you will need to have another dose at least 21 days after your previous dose. There is no maximum time limit between doses, so you do not need to repeat the first dose or receive a third dose.

If you received one dose of different vaccine (for example, COVID-19 vaccine AstraZenaca/Vaxzevria/Covishield or the Moderna mRNA COVID-19 vaccine), the current recommendation is to have one dose of the Comirnaty vaccine when it becomes available for your priority group at least 4 weeks after your first vaccine. These vaccines are not interchangeable, but you are likely to have a good response to just one dose. If you received one dose of the Janssen/Johnson & Johnson vaccine, you are considered fully immunised and do not require any further doses of COVID-19 vaccine.