From early January, New Zealanders who have had two doses of the Covid-19 vaccine for at least four months can get their booster dose.

The timing of the rollout had previously caused some confusion about when people should go and how it all works.

There's also a difference between booster doses and third primary doses.

Below are answers to come frequently asked questions.

What is the booster dose?

The Pfizer vaccine is being used in New Zealand for boosters, regardless of what vaccine was used for earlier doses.

The booster is different to the third primary dose recommended for people who are immunocompromised.

People eligible for a third primary dose can access a booster dose six months after receiving their third primary dose.

When can you get your booster?

In early December, the government announced it would shorten the required gap between the second dose and booster, from six months to four months.

For people who don't mind going to a walk-in vaccination centre, they are able to get your booster dose from 5 January. Find locations by clicking here.

For those who prefer to book in, appointment slots will be available from 17 January via Book My Vaccine online.

While the rollout with shortened intervals officially starts in early January 2022, people who have had their second shot for six months were encouraged to go get their booster dose right away.

Others wanting to get in early before 5 January were advised to check with their local vaccination provider to ensure they have appointments and supply available.

The Ministry of Health has said there were plenty of booster doses available and no one would miss out.

Other options for booking

If you cannot book online, you can call the Covid-19 Vaccination Healthline on 0800 28 29 26 (8am to 8pm, seven days a week).

Interpretation services, and text, email and NZ Relay options for deaf and hearing impaired are available if you need them via the 0800 number.

There is also a specialist team for disabled people (option 2 on the 0800 number).

You can also make an appointment with your doctor if they are providing Covid-19 vaccines.

Who is eligible for the Covid-19 vaccine booster?

Healthcare and border workers have been the priority for booster doses since large numbers of people in those industries had their primary doses at least six months ago.

When you arrive for your booster, the date of your last dose will be checked in the booking system to ensure you have gone through the required interval period.

People aged 12 years and older have so far been able to get their primary Covid-19 vaccine courses throughout 2021, but the booster dose is currently only going to be available for those aged 18 years and above.

(For most people, a primary course is two doses, for some people a primary course could be three doses.)

If you had your full 'primary course' while you were pregnant, the Ministry of Health advises you to wait until after your baby is born before you take a booster.

Those who had their full 'primary course' before they became pregnant only need to wait six months on before taking the booster, the ministry said.

If you had your Covid-19 vaccination overseas, you can get still get the Pfizer booster once you have gone through the required interval from your last dose.

Potential side effects of boosters

You may experience some side effects with the booster similar to that of the second dose of the Pfizer vaccine, according to the Ministry of Health.

These include muscle aches, pain at the injection site, headaches, nausea, and feeling tired or fatigued. For most people, these tend to be mild effects and don't last long.

Some side effects are more serious but very rare, like a severe allergic reaction or an inflammation of the heart (myocarditis).

An online reporting form for adverse events following immunisation with Covid-19 vaccines is now available.

If you develop difficulty breathing, a racing heart, chest pain or feel faint immediately or in the days after the vaccine, you should seek medical attention.

If you're unsure about your symptoms or if they get worse, call Healthline: 0800 358 5453.

Vaccine mandates and vaccine passes for boosters

You do not need to have a booster to be certified as 'fully vaccinated' for My Vaccine Pass or an International Travel Vaccination Certificate.

If you do get a booster dose, it will be added to My Covid Record and you can create another pass.

Early in December, the government announced Cabinet has agreed in principle that where workers are required to be vaccinated, this mandate will now extend to booster doses.

Border and health workers will be required to have their booster dose by the end of January, or not later than six months after their second dose for those recently vaccinated.

It will then be extended to all others covered by vaccination mandates from 1 March. These changes are expected to be confirmed in January 2022.

How effective is the booster against Covid-19?

The Ministry of Health says data from Pfizer shows that a booster dose is 95.6 percent effective against the coronavirus, including the Delta variant, compared to those who did not receive a booster.

At this stage, there is no data available on duration of protection against infection and disease following a booster dose.

The ministry said current evidence shows that after six months of a full vaccination course, protection against Covid-19 is reduced and so a 'top up' vaccine will help boost immunity against the virus.

"We already know that booster vaccinations significantly lift an individual's immunity, reducing the spread and the severity of Covid-19.

"Data is emerging now that booster doses of Pfizer provide better protection than two doses do when it comes to the Omicron variant."

The UK Health Security Agency (UKHSA) analysed more than 600,000 confirmed and suspected cases of the Omicron variant up to 29 December in England and found a booster vaccine is 88 percent effective at preventing people ending up in hospital with Covid-19.

The new data confirms that two doses of the AstraZeneca, Pfizer or Moderna vaccines offers little protection against being infected with Omicron.

Why has the time between the second dose and booster been shortened?

Covid-19 Response Minister Chris Hipkins has said that soon it is expected that every case coming into New Zealand's border would be of the highly infectious Omicron variant.

The government hopes having widespread administration of the Covid-19 vaccine booster will help prevent any potential massive outbreak of the variant and decrease the risk of transmission.

Director-General of Health Dr Ashley Bloomfield said speeding up the booster rollout was a key part of the country's response to Omicron.

"The shorter interval means more than 82 percent of vaccinated New Zealanders will be eligible for a booster by the end of February 2022."

-RNZ.