April 19, 2021
Every Monday, we’ll answer your questions on Covid-19 and health in a feature published online. You can submit a question here.Dozens of HuffPost UK readers have shared questions and concerns about the second dose of the AstraZeneca vaccine. 
What To Know Before Your Second Dose Of The AstraZeneca Vaccine
Since rumours surrounding the AstraZeneca jab and a possible link to blood clots began swirling, there has been some worry about what that means for those who’ve already had their first dose of the vaccine. 
“If I’ve had my first AstraZeneca vaccine, will I be at risk of getting a clot on my second?” asks HuffPost UK reader Ashleigh. While reader Jacqueline asks: “I had the AstraZeneca jab in February and was very ill for over a week and still don’t feel well. I am reluctant to have the second dose. Could I have the Pfizer?”
We looked into people’s questions and concerns, with help from the Medicines & Healthcare products Regulatory Agency (MHRA). Here’s what you need to know.Submit a Coronavirus health question to HuffPost UK.Do I have to have the AstraZeneca jab as my second dose?As it stands, if you have the AstraZeneca vaccine as your first dose, you need to have the second dose from the same manufacturer (except in rare instances, which we’ll come to shortly). The second dose should be issued between four and 12 weeks after the first.
The reason why you can’t pick and choose the vaccines is because we don’t know how safe it is to mix and match them – or how well they work together.
Trials are underway to explore this. The Com-Cov study, which launched in February, is investigating alternating doses of the Oxford/AstraZeneca, Pfizer, Moderna and Novavax vaccines. Led by the University of Oxford, the study is recruiting adults aged 50 and over who have received their first vaccination in the past 8-12 weeks.For the purpose of science, you would be able to get a different type of vaccine issued as your second dose. Volunteers can sign up here. Are there any circumstances when you shouldn’t have the second dose?There are some instances where you wouldn’t be advised to have the second dose of the AstraZeneca vaccine, even if you had it as your first. In these cases, you would be offered an alternative by Moderna or Pfizer.
People shouldn’t have the second dose if they have hypersensitivity to the active substance in the jab – Covid-19 vaccine (ChAdOx1-S recombinant) viral particles – or to any of the following ingredients: L-histidine, L-histidine hydrochloride monohydrate, magnesium chloride hexahydrate, polysorbate 80, ethanol, sucrose, sodium chloride, disodium edetate dihydrate.
Of course, you might not know if you’re allergic to any of these ingredients as they’re pretty niche. Here, the best advice is that if you’ve had a systemic allergic reaction (including anaphylaxis) to a previous dose or any component of the AstraZeneca vaccine, you shouldn’t receive the second lot. If you haven’t, you’re fine to go ahead with your second dose as normal.
Patients with a history of heparin-induced thrombocytopenia and thrombosis (HITT or HIT type 2) should also have a different second dose, according to government guidance. As a precautionary measure, those with a history of cerebral venous sinus thrombosis or antiphospholipid syndrome should speak to their doctor about the potential risks associated with having the jab before going for their next dose.
In very rare cases, some people have experienced blood clots with low platelets after having the first dose of the AstraZeneca vaccine. These people should be given a different second dose. It’s worth noting the risk of developing a blood clot after the first dose is extremely low – in fact, if you develop Covid, you’re far more likely to have a blood clot compared to having this particular vaccine. What’s the risk of getting a blood clot after the second dose?The risk of developing a blood clot with your second dose of the vaccine is even more rare. Up to 5 April, the MHRA had heard of 100 cases of blood clots with low platelet counts in the UK following vaccination with the AstraZeneca jab. 
Of these, 99 reports were associated with the first dose of the vaccine and a single report followed a second dose, although this individual had medical conditions that could have caused the events. For context, this is out of 20.6 million first doses and one million second doses of the vaccine issued in total.Are side effects worse the second time around?Anecdotally, side effects appear to be more common with the AstraZeneca vaccine than others. Data shows that as of 5 April 2021, the MHRA had received 46,309 reports of suspected side effects (also known as Yellow Cards) for the Pfizer/BioNTech vaccine and 129,673 reports for the Oxford/AstraZeneca vaccine.
That said, side effects reported with second doses of the vaccine appear to be milder and less frequent. So the chances are your second jab won’t be as bad. 
Experts are still learning about Covid-19. The information in this story is what was known or available at the time of publication, but guidance could change as scientists discover more about the virus. To keep up to date with health advice and cases in your area, visit gov.uk/coronavirus and nhs.uk.Related...What Happens When You Mix The Covid-19 VaccinesHere's Where We're At With Covid Booster Jabs For AutumnHow The Vaccine Rollout Could Look Between Now And JulyShould You Take Aspirin Before The Covid Jab? We Ask Experts
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