AstraZeneca and Pfizer vaccines - FAQs for Young Adults


Marylouise McLaws: relative risk of blood clotting from AstraZeneca

To help you navigate the information and advice on the COVID-19 AstraZeneca and Pfizer vaccines and ensure you have access to up-to-date evidence-based information, we asked UNSW expert epidemiologist, Professor Mary-Louise McLaws to answer your most frequently asked questions on the safety and efficacy of the vaccines in young adults.

COVID -19 Vaccines Frequently Asked Questions 

The FAQs below have been developed in collaboration with ARC and Professor Mary-Louise McLaws, epidemiologist with expertise in hospital infection and infectious diseases control, Advisor to the World Health Organization Health Emergencies Program Ad-hoc COVID-19 Infection Prevention and Control Guidance Development GroupInformation is correct as at 13.08.2021. 

For detailed and up to date guidance on COVID-19 vaccines, and to book a vaccination appointment, visit the NSW Health website. 

The risk of developing serious disease or dying from COVID-19 in young adults with no underlying health conditions is low but unpredictable. A meta-analysis study shows the fatality rate increases with age. At 10 years old it is 0.002%, at 25 years it is 0.01%, and 0.4% at 55 years of age (Levin, 2021).

Studies have shown that vaccines are highly effective across all age groups in preventing symptomatic disease, transmission, hospitalisations, and mortality (Bernal, 2021).

The current Delta outbreak in NSW, and experience in the UK, demonstrates that COVID-19 is largely a disease of the unvaccinated.

The rate of long-term health consequences associated with COVID-19 (termed ‘Long COVID-19’) is much higher than the rate of death. The prevalence of long COVID-19 was estimated in the UK to be 21% at 5 weeks and 10% at 12 weeks from onset (Office For National Statistics (UK), 2020). The definition of Long COVID-19 is still being developed, but the common characteristics include symptoms of:

  • unpredictable fatigue
  • headache
  • difficulty breathing
  • loss of taste and smell
  • lasting brain fog (Sudre, 2021).

Data from the UK shows those aged 5-24 have a higher prevalence of infection than any other age group – the risk in this age group was many were unvaccinated (Riley, 2021).  In Australia, young adults aged 20-29 have the highest prevalence of infection. (Australian Government, Department of Health, 2021) In NSW those aged 0-39 years represent 68% of all Delta infections (NSW Health).

Vaccines protect the people from Delta strain severe disease, hospitalisation, and death.

The first dose reduces the risk of severe illness and its complications but provides little reduction in symptomatic disease (30-36% reduction) with the Delta strain. The second dose dramatically reduces the infection-transmission cycle by 67-88% (Stowe, 2021). Meaning, if you are fully vaccinated you have good protection against symptomatic infection, and in very rare cases of a “break though infection” (infection even after being fully vaccinated) you are more likely to have milder infection.

The benefits from the first dose of vaccine include a reduction in the risk of severe disease and a reduction in the risk of hospitalisation. Clinical trials have shown that having two doses of the vaccine is essential for ensuring strong, longer-lasting immunity against COVID-19. The first dose of the vaccine activates your immune system and the second dose boosts your immune response giving your body a greater chance to fight COVID-19

Memory T cells can remain in the blood stream for months and immunity can last decades to some pathogens. It is not yet known how long vaccination will protect against COVID-19 and we may need regular booster vaccinations to ensure our immunity remains high. You are more likely to have longer protection against severe disease than protection against infection.

  • The original dose interval, between dose 1 and 2, for Pfizer was 4 weeks and 12 weeks for AstraZeneca.
  • WHO recognises the need for altering dosing interval in extraordinary circumstances
  • In Australia the dosing interval for Pfizer has been extended to 6 weeks and shortened to 4-8 weeks for AstraZeneca to maximise population coverage especially during the Delta outbreak.
  • Post vaccination immunity is highest 14 days after the second dose of both vaccines. Before this time, you may still be able to catch and transmit COVID-19.

Continued coverage in the media of the links between blood clots and the COVID-19 vaccine causes a cognitive bias and increases our perception that the risks associated with having the vaccine are greater than they really are. Let’s put the risk into perspective:

Risk of developing a blood clot

First dose of AZ: the risk of a blood clot is classified as ‘very rare’ (Department of Health, 2021):

  • 1.9 per 100,000 for people aged 18-29
  • 1.6 per 100,000 for people aged 30-39

Second dose of AZ: the risk of a blood clot is significantly reduced; in fact, it’s not significantly different to the risk in unvaccinated people (Bhuyan, 2021):

  • 1 in 1,500,000

The risk of a blood clot from AZ is 8-10 times lower than the risk of a blood clot during a COVID-19 infection (Taquet, 2021)

Risk of a blood clot compared with other activities

The risk of developing a blood clot from AZ vaccine is far lower than many other medicines and activities for example:

  • Taking the combined contraceptive pill 1:5,000 (Therapeutic Goods Administration, 2016)
  • Taking a long-haul flight 1:60,000 (Zarocostas, 2007)

Risk of dying from a blood clot

With improved management and probably earlier detection of severe blood clots the risk of dying from a blood clot associated with the AZ vaccine in Australia is low at around 5-7%. The overall risk of dying from a severe blood clot is extremely low.

  • Risk of death from a blood clot following an AZ vaccine: 1:2,000,000
  • Risk of death by other causes
    • Struck by lightning: 1:500,000
    • Shark attack while scuba diving: 1:10,000
    • Killed by a cow: 1:1,000,000
    • Bee sting (Remember the bees on campus!): 1:59,507
    • Champagne cork accident: 24 deaths per year globally
  • To end the comparisons on a happier note (or not!) compared with a blood clot from AZ you have a better chance of:
    • Winning gold at the Olympics: 1:662,000
    • Writing a New York Times best seller: 1:220

Common side effects of the COVID-19 vaccine, aside from being a bit uncomfortable at the injection site, aren’t a cause for concern. These include: fever, sore muscles, tiredness and headache. These symptoms usually start within 24 hours of receiving the vaccine and ease after one to two days. (AstraZeneca Australia, 2021)

Blood clots are a very rare, serious side effect and when caught early, they can be treated effectively, the chances of survival are high (Health Direct, 2021). A blood clot associated with the AZ vaccine is usually apparent within 21 days of the first dose of the AZ vaccine. However, you should monitor for symptoms for up to 42 days post-vaccine and seek medical advice immediately if you have any of the following:


All vaccines can have side effects and the most common ones, like those associated with Pfizer are pain or swelling at the injection site, tiredness, headache, muscle pain, fever and chills. These usually appear within 24 – 48 hours and last 2- 3 days. Unlike AZ which has similar side effects predominantly after the first dose, the side effects after Pfizer are more commonly experienced after the second vaccine. 

Mild myocarditis has been reported, but no plausible causal link has yet been identified (World Health Organisation, 2021).

There is good evidence from Qatar and the UK for reduction in adverse outcomes associated with COVID-19 Delta strain after being vaccinated with either AZ or Pfizer:

  • Reduction in hospitalisations:
    • 77% after the first dose of AZ and 92% after the second dose
    • 76% after the first dose of Pfizer and 96% after the second dose (Gower, 2021)
  • Reduction in symptomatic infections:
    • 30% after the first dose of AZ but 60% after the second dose (Canada)
    • 36% after the first dose of Pfizer but 79% after the second dose (Scotland) (Sheikh, 2021)
  • Reduction in mortality:
    • 85% for both AZ and Pfizer (Public Health England, 2021).

Note: These data sets have not been analysed to control for differences in testing protocols for when a participant is tested for COVID-19, population age, vaccine coverage, underlying health risk factors.

The very rare risks of having a serious side effect associated with AZ vaccine haven’t changed. What has changed is the relative risk of catching the Delta strain which is currently circulating in Australia. Public Health England warns us that Delta is 50%-60% more infectious than the Alpha strain. It is more contagious with those infected having an average viral load 1000 higher than seen with previous strains (Li, 2021) making it easier to pass it on to others who may develop serious or long-term health impacts and/or death.

In this third wave we are currently experiencing in Australia, the benefits of having the vaccine are greater than the risks of the rare side effects of AZ or Pfizer vaccines for all age groups. This led the government to change its advice for young people around AZ (Australian Government, Department of Health, 2021).

The benefit of reduced risk of serious illness and symptomatic disease to yourself (see vaccine efficacy above) and the community is compelling.

In Greater Sydney, everyone 18 years of age and over is encouraged to get vaccinated with any available vaccine.

You can call the National Coronavirus Helpline on 1800 020 08 or send an enquiry. You can also make an appointment at the UNSW Health Service.

There is an online eligibility checker which you can use to find out when and where you can book for a vaccine. If your preferred vaccine is not available, you can set up alerts to be notified once it becomes available.

Alternatively get to know where the walk in vaccine clinics have opened up in Sydney, where you can go without a booking.

If you are 16 -39 years old and live or work in one of the 12 local government areas of concern, you can book a priority Pfizer vaccination appointment here.

Absolutely! The Australian Government is providing free COVID-19 vaccines to everyone currently in Australia, and you don’t need a Medicare card.

It is a tough topic to approach; the key things would be to listen and acknowledge their concerns and ask questions. Encourage them to be consider how reliable their sources of information are and provide them with evidence-based alternatives to consider, try not to be pushy or confrontational.

World Health Organization has some excellent resources for talking about vaccines with those who may be hesitant or unsure about having a vaccine. Tell them when you have had your vaccine because this may assure them that it’s the safest option.

Yes! Having the vaccine means that it is harder for you to catch and transmit COVID-19. You are gifting health to yourself and protecting your unvaccinated close contacts too.

Once enough of the population are vaccinated, there are potential socioeconomic benefits such as

  • Increased social mobilisation (for example, being able to attend large gatherings, birthday celebrations, concerts, gigs etc.)
  • Freedom from lockdown or school closures
  • International travel
  • Financial stability for those working casual jobs e.g. hospitality etc.


and “getting back to the good stuff” safely!