HuffPost UK reader Safia asked: “I received my first dose of the AstraZeneca vaccine on 17th April. It is well over one month today – when should I be getting my 2nd dose?”

With millions of first doses of the Covid-19 jab administered already, those who haven’t got their second one booked in are wondering when they’ll get it. 

If you book your Covid jab on the NHS website, you should be able to schedule your first and second dose at the same time. However, if you get invited by your GP – or had your first jab earlier in the year – you might be waiting to be invited back for your second dose. Here’s what you need to know if that’s the case.

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What are the dosing schedules for the jabs?

The Pfizer vaccine is given as two doses, ideally a minimum of 21 days apart, while the Moderna vaccine doses should be administered a minimum of 28 days apart.

Two doses of the AstraZeneca vaccine should be given a minimum of eight weeks apart. But in cases where rapid protection is required – for example in people due to receive immunosuppressive treatment – it’s possible for them to be given two doses closer together (28 days apart).

To avoid confusion and simplify the booking process, the NHS is following a dosing schedule of around 12 weeks for all vaccines so more people can get the benefits of the first dose. 

There is no reason to suggest this is a problem for those vaccines like Pfizer and Moderna, which should be given about three to four weeks apart.

In fact, a preprint study led by the University of Birmingham in collaboration with Public Health England (PHE) found the antibody response in people aged 80 and over is three-and-a-half times greater in those who had their second dose of the Pfizer jab after 12 weeks compared to those who had it after three weeks. 

On May 14, the Joint Committee on Vaccination and Immunisation (JCVI) recommended that people in priority vaccine groups be given their second dose after eight weeks rather than 12, to give them full protection faster. 

What happens if you’re not called up for your second dose? 

HuffPost UK reader Safia suggested she’d called the pharmacy that administered her first dose but they weren’t able to offer her a date for the second. In this instance, people can go online and rebook their second dose through the NHS national booking service. 

If anyone was invited for their first dose by their GP or a hospital hub, they should be contacted about their second dose. HuffPost UK understands that pharmacy and vaccine centres aren’t contacting people about their second doses however, so if you don’t hear anything, it’s best to book online.

The good news is that if you’ve moved house or you need to have your second dose at a different location, you can opt to do that when you book. 

Some people might be worried if there is a longer interval between their two doses than the recommended 12 weeks. In this instance, the second dose should still be given and it should be the same vaccine type as the first. The course does not need to be restarted.

There’s no reason to suspect the vaccine will be less effective if the second dose is given beyond 12 weeks. However, the second dose is what completes the course and is likely to be important for providing you with longer term protection – so it’s important you have it.

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.