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Explainer: COVID-19 vaccine dosage

As it stands, over 4 million doses of COVID-19 vaccine have been administered in Australia. This doesn’t mean 4 million people are fully vaccinated – both of the vaccines currently approved for use in Australia require two doses to reach full immunity. As of the end of May, half a million Australians have been fully vaccinated against COVID-19.

Additionally, some people who have received one vaccine dose have tested positive to COVID-19 in Melbourne this week.

So why do these vaccines need two doses – and what’s the logic behind the timing between doses?

Why do we need two doses to be fully vaccinated?

The aim of a vaccine is to provoke an immune response in the body. For lots of vaccines, the best way to do this is to provoke the immune system twice.

“We call it prime, and boost,” says Stephanie Gras, a professor of immunology and medical science at La Trobe University.

“Your first vaccination dose is your priming of the human system, so you’re actually teaching your immune system to recognise this protein from the virus.

“The second one is the boosting effect, where your immune system knows about it, but now you want your immune system to be really strong. That’s why the two doses are really important.”

The number of antibodies is particularly important as new variants of SARS-CoV-2 emerge.

“New studies show that even if the Indian variant can spread faster, the current vaccines that we have in Australia (Pfizer and AstraZeneca) have strong efficacy against the variant and so people who are vaccinated are protected,” says Gras.

“The data show that even if the antibody levels are decreased due to the variation in the virus, the vaccine still provides a strong level of protection. It is also important to note that other immune cells such as the killer T cells are very good at recognising different variants and protecting us.”

At the moment, all approved mRNA vaccines require two doses, and so do most COVID-19 vaccines that use other methods.

Why do some vaccines only require one dose?

The Johnson & Johnson vaccine, which is being used in some countries (like the US), only requires one dose to be effective. Interestingly, it uses the same method as the AstraZeneca vaccine to provoke immunity: an adenovirus vector, containing spike proteins from COVID-19.

Gras says that while different vaccines may use the same technique, they’re made by different companies and tested in separate trials, so they’ll still have different ingredients. This means they’ll provoke different levels of immune response. “Even if some of them are based on the same platform and the technology behind is the same, they’re actually not the same vaccine,” she says.

“It’s like you’re making a chocolate cake. At the end it’s going to be a good chocolate cake, but every chef has a different recipe.”

Does one dose of a two-dose vaccine protect me from COVID-19?

Yes, but not as much as two doses – and the level of protection depends on both the vaccine being used and the variant of SARS-CoV-2.

“The vaccine provides protection after two weeks of your first injection,” says Gras. “The thing is that the level of protection is different between the vaccines.

“Something also to keep in mind is that all the vaccines have been developed against the Wuhan strain of the virus.”

Variants – such as the B1.617 strain currently in Melbourne – have different resistance to vaccines.

For instance, a study with over a million participants in Israel found that the Pfizer vaccine was 46% effective at preventing COVID-19, 14 to 20 days after the first dose. Over the study period, around 80% of infections in Israel were due to the B1.1.7 strain first detected in the UK. (In this study, the vaccine was shown to be 92% effective after the second dose.)

The strain currently in the Melbourne community is newer, but one dose of a vaccine is still likely to have some effect – although two doses is much more effective. Gras says that after one dose, the Pfizer vaccine efficacy is “actually down to about 30% for the Indian variant, which could explain why we may have some people who are able to have COVID, even if they have been vaccinated.”

“If they’ve been vaccinated just a few days before catching the virus, that probably won’t give enough time to the immune system to actually build a response,” she adds.

“Reports from the UK also show that the effectiveness of one dose of Pfizer or AstraZeneca is only 33% against B1.617.2; but this rises to 88% after two doses for Pfizer and 59.8% after two doses of AstraZeneca,” says Raina MacIntyre, head of the biosecurity program at the Kirby Institute of the University of NSW.

“The good thing is that after the second dose, the vaccines are really efficacious against the variant,” says Gras.

How long is the wait between doses?

The type of vaccine informs the time required between doses.

“Some vaccines will stimulate the immune system to react fast,” says Gras. “And for some of them, over a longer period of time you will build a stronger immune system for the vaccine, which means you can actually delay the second dose.”

For the Pfizer vaccine, the second dose should be given about 21 days, or three weeks, after the first. Clinical trials of the Pfizer vaccine used this approximate interval, and found it highly effective.

The AstraZeneca vaccine has a longer and more flexible timeframe for its second dose. According to the Health Department’s website, AstraZeneca’s second dose can be given 4–12 weeks after the first dose. A number of studies have shown that it will be safe and effective anywhere in that timeframe.

ATAGI, which guides the government on the vaccine rollout, has recommended that the AstraZeneca vaccine be given 12 weeks apart. This will “ensure the most effective clinical protection and maximise broader community coverage.”


You can learn more about ATAGI in our Cosmos Weekly article. Subscribe to read the full piece.


Because supplies are limited, one of the priorities of the rollout is to get more people partially vaccinated rather than a few people fully vaccinated. This means preference has been given to space AstraZeneca doses at the maximum 12 weeks apart, rather than four, because then more people can have time to receive their first dose.

“If this interval is not possible, for example because of imminent travel, cancer chemotherapy, major elective surgery, a minimum interval of four weeks between doses can be used,” notes the TGA on their website.

Can I mix vaccines?

Could you get one dose of Pfizer, and one of AstraZeneca? We published a longer explainer on mixing doses recently, and we recommend you read that for more information.

The short answer: there’s some early evidence from overseas suggesting that mixing vaccines is okay, but it’s certainly not approved or recommended by Australian health authorities yet. It’s unlikely anyone in Australia will have the opportunity to try this, anytime soon.

As always, you should chat to your doctor if you have any personal health concerns or questions.