The Government will make the AstraZeneca Covid-19 vaccine available for a small group of people, officials have announced.

New Zealand has an advance purchase agreement for 7.6 million doses of the Oxford/AstraZeneca vaccine – one of four the Government put its hand up for, alongside Pfizer/BioNTech, Janssen and Novavax.

Medsafe provisionally approved the vaccine in July but it has remained off the shelves until now.

So how does it work and how is it different from the Pfizer vaccine at the centre of New Zealand’s roll-out?

How it works

The AstraZeneca vaccine (also known as Vaxzevria) is a two-dose, viral vector vaccine, developed by Oxford University and British-Swedish pharmaceutical company AstraZeneca.

There are a number of vaccine types, including viral (live or inactivated), viral vector, subunit or DNA/RNA vaccines.

Vaccines contain weakened or inactive parts of an organism (antigen) that trigger an immune response within the body. If the body encounters the real antigen later, it already knows how to defeat it.

The technology used for the AstraZeneca vaccine is similar to the Janssen vaccine, which has also been provisionally approved in New Zealand.

The AstraZeneca vaccine is made up of another virus, an adenovirus – which can enter human cells but cannot replicate or cause illness – and the code to make the SARS-CoV-2 spike protein.

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After entering our cells, the modified adenovirus tells the body how to produce copies of the Covid-19 spike proteins, which activates the immune system.

If exposed to the real Covid-19 virus, the immune system can then block the virus from entering the body’s cells to protect it from illness, the Immunisation Advisory Centre (IMAC) – run out of the University of Auckland – states.

AstraZeneca does not contain any live SARS-CoV-2 virus, so you cannot contract Covid-19 from the vaccine.

It can be stored at normal fridge temperatures (2 degrees Celsius to 8C) for up to six months.

Who can access it and when

Announcing the move on Wednesday, Director-General of Health Dr Ashley Bloomfield indicated the vaccine was likely to be available for those aged 18 and over in late November.

“Quite a lot of work” was needed to ensure the workforce was trained to use a second vaccine; that clinics could accommodate another vaccine; and that there were appropriate cold store and logistics in place, he said.

It will not be available for everyone.

It will be offered to the fewer than 100 New Zealanders who cannot have the Pfizer vaccine due to medical reasons, such as a severe allergic reaction.

Bloomfield said he anticipated only a “few” hundred people would fit the criteria.

It will also be available for those who prefer not to have an mRNA vaccine, if their job requires them to be vaccinated.

There will be a process around accessing the vaccine, which officials expect to make announcements on next week.

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It will also only be available in a “limited” number of places but there will be geographic spread across the country for those who need it.

What we know about efficacy, safety and side-effects

Data shows AstraZeneca is both safe and effective, providing protection against severe illness and death.

Trials showed it was 55-81 per cent effective against symptomatic (Alpha) Covid-19, IMAC states.

Findings of a large-scale United Kingdom study published in August showed AstraZeneca was effective against Delta after two doses but – like with Pfizer – protection waned over time.

It found Pfizer was 92 per cent effective at keeping people from developing high amounts of the virus in the body 14 days after the second dose. For AstraZeneca, this was 69 per cent.

A Public Health England study found AstraZeneca's level of protection against hospitalisation 20 weeks after vaccination was 77 per cent, contrasted with 92.7 per cent with Pfizer.

Common adverse events for AstraZeneca are similar to those reported with Pfizer: including mild arm pain and tenderness, fatigue, headache, sore muscles and fever.

There is a very rare but serious effect of blood clots (thrombosis with thrombocytopenia syndrome, or TTS) linked to the AstraZeneca and Janssen vaccines.

This is typically seen in those aged under 50 years, following the first dose.

IMAC states the incidence of TTS is very rare, with one to <15 cases per million doses, depending on age.

Covid-19 itself is associated with a much higher risk of blood clots – roughly more than one in five cases.

Different countries have rolled out the vaccine to different age groups taking this into account.

In Australia, which rolled out AstraZeneca as its primary vaccine in February, the risk of developing TTS after a first dose of the vaccine is about 20 in a million. TTS can be treated and most people recover.

The risk of dying from TTS after a first dose of AstraZeneca is less than one in a million – versus the risk of dying if you catch Covid (42,000 in a million), the Australian Government's department of health states.

While the vaccine is approved in Australia for those aged over 18, the Australian Technical Advisory Group on Immunisation recommends it for those aged over 60. It is recommended for under 60s in the event of an outbreak when Pfizer or Moderna are not immediately available.

Bloomfield stated that all vaccines had advantages and disadvantages, and benefits and risks.

The Pfizer mRNA vaccine's effectiveness and safety profile were “very good”. AstraZeneca, however, was not as effective against the Delta variant and there were some specific rare side-effects, he said.

“We want to make sure that anyone deciding to have the AstraZeneca vaccine has access to that information just as we have provided for the Pfizer one,” he said.