COVID-19: Vaccine effectiveness and protection

Find out how effective the COVID-19 vaccine is and how it protects you. Including how efficacy was measured, why you need a second dose and transmitting COVID-19.

Last updated: 18 November 2021

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This page is about the Pfizer vaccine. For information about AstraZeneca, see AstraZeneca vaccines.

How effective is the COVID-19 vaccine, and what does 95% mean

As with any vaccine, the Pfizer vaccine (Comirnaty) may not fully protect everyone who gets it. However, it is highly effective if people have both doses. That means, if you do catch COVID-19, you’re far less likely to fall seriously ill and less likely to transmit the virus to others.

Clinical trials found that the Pfizer vaccine gave 95% protection against the symptoms of COVID-19.

The COVID-19 vaccine stimulates your body’s immune system to produce antibodies and other proteins that will fight the virus if you’re exposed to it. This reduces the risk of getting infected and if you do get COVID-19, it means you could have no symptoms or will have much fewer, milder symptoms and recover faster.

While the data is clear that vaccines protect people from the effects of COVID-19, research is ongoing to determine whether a vaccinated person could still transmit the virus to someone else – so to be safe, we must assume there is still a risk of transmission.

The difference between efficacy and effectiveness

Efficacy is the measure used in clinical trials

Efficacy measures how well a vaccine can prevent symptomatic infection (and sometimes transmission) in clinical trials. This is under ideal and controlled conditions, comparing people who receive the vaccine with those who receive a saline placebo.

Effectiveness is the measure used in the real-world

Effectiveness is how well the vaccine performs in the real world outside of the clinical trials in a mixed population. We would expect a vaccine with a high efficacy to be highly effective in the real-world but these measures are unlikely to be the same.

How efficacy was measured

The efficacy of the Pfizer vaccine (Comirnaty) was measured in two ways.

Phase 1 clinical trial – level of antibodies

The immune response to the vaccine was measured by looking at the level of antibodies in the bloodstream and how well they worked to neutralise the COVID-19 virus in laboratory tests.

Phase two and three clinical trials – vaccine and placebo

The efficacy of the Pfizer vaccine was tested in about 44,000 participants aged 16 years and over where COVID-19 was already circulating in communities. About half of these participants were randomised to receive the vaccine and the other half received a saline placebo.

The trial looked at how many people got COVID-19 symptoms after they were vaccinated compared to how many got COVID-19 after getting the placebo.

Participants had two doses of the vaccine or placebo, getting their second dose within 19 to 42 days after their first dose. They were then closely monitored and evaluated for at least 2 months after their second dose.

Vaccine clinical trials and testing

Efficacy across different groups

A consistently high efficacy (between 90 and 100 percent) was observed in the clinical trials across age groups, sex, race, ethnicity and people with underlying medical conditions.

This means after getting two doses of the Pfizer vaccine, more than 9 out of 10 people are protected against COVID-19 regardless of their age, health status or ethnic group.

Efficacy in young people

Following Medsafe Provisional Approval, the decision to vaccinate young people aged 12–15 years has been approved by Vaccine Ministers, Health Officials and Cabinet.

Pfizer’s study in 12- to 15-year-olds looked for signs of a strong immune response to the vaccine. Pfizer reported 100 percent efficacy in this age group (a higher antibody response than those studied in the 16- to 25-year-old age group).

Long-term efficacy

To understand the long-term efficacy and safety of the vaccine, participants in the clinical trials are being tracked for another two years after their second dose of the Pfizer vaccine.

Getting your second dose increases protection

Clinical trials showed the Pfizer vaccine (Comirnaty) had a higher efficacy against symptomatic COVID-19 infection after receiving the second dose. This is supported by recent real-world data.

The first dose ‘primes’ your immune system but protection doesn’t last as long because the level of antibodies falls. A second dose gives your immune response a boost – with lots more antibodies to help your immune response to mature and provide longer protection.

Why you need two doses

If you’re delayed getting the second dose

The recommended time between the two doses is 3 weeks or more. You won’t have the full protection of the vaccine until you’ve received your second dose. If you get your second dose later than this you'll still be fully vaccinated – you don't need to restart the vaccine course.

How long you’ll be protected for

We don’t yet know how long you’ll be protected but will continue to review data as it becomes available.

Current evidence suggests that after two doses protection against severe disease, including hospital and ICU admission, remains high, including for the Delta variant, but that antibody levels decline over time.

Catching and passing on COVID-19

The Pfizer vaccine (Comirnaty) is effective at reducing the number of people who get COVID-19.

It’s harder to find out how well the vaccine stops people passing on (transmitting) the COVID-19 virus. Recent studies show that the Pfizer vaccine can reduce transmission of the virus. These studies looked at the number of people infected with COVID-19 after they’d been vaccinated and their close contacts.

COVID-19 vaccines and their effect on viral transmission (PDF, 257 KB)

Keep taking precautions

To help protect yourself, your whānau and others, continue taking precautions to prevent the spread of COVID-19 after you’ve been vaccinated.

Protecting yourself and others from COVID-19

Testing after you’ve been vaccinated

As there’s still a risk of transmission, it’s important to go and get tested if you have COVID-19 symptoms, even if you’ve been vaccinated.

The sooner you know you have COVID-19, the faster you can act to protect yourself, your whānau and your community.

Mandatory COVID-19 testing of our border and MIQ workers will also continue.

The vaccine is given by an injection with a needle in the upper arm and takes two doses at least three weeks apart. You cannot get COVID-19 from the vaccine, as with any vaccine this one may not fully protect all those who receive it and it will take a few weeks for your body to build up protection.

After you have had the vaccine you still need to follow official advice to help prevent the spread of COVID-19.

Before having the vaccine a healthcare professional will run through your medical history to ensure it is safe for you to have this vaccine.

Some side effects that you might experience include pain where the vaccine was given, tiredness, headache, muscle aches, joint pain, chills, mild fever and or swollen glands.

These are usually mild in severity and only last for one or two days. Generally these are more common after the second dose of the vaccine. You can take pain medications such as paracetamol to ease these symptoms. Now as with any medicine allergic reactions are possible but very rare.

Current evidence suggests a rare risk of a severe allergic reaction with this vaccine - about five per million. This is why we ask you to stay for 30 minutes after you have been vaccinated.

So COVID-19 vaccines are being held to exactly the same high safety standards as any other vaccine there are many reasons why these vaccines were able to be made more quickly while still being held to the same high safety standards.

First there has been significant investment internationally in the development of these vaccines including taking financial risks such as building manufacturing facilities even before a vaccine has been shown to work.

Researchers policy makers and government officials around the world have been sharing information and working together to absolutely speed things up. There's been decades of previous research on related viruses that has been used to guide the development and finally because there is such a large amount of this disease worldwide at the moment researchers were able to show that the vaccine worked much earlier than when dealing with rarer diseases.

The clinical trials performed on the Pfizer vaccine show it is approximately 95% effective after receiving two doses. The vaccine is less effective after only one dose.

Well we don't yet have the final answer for that because the vaccines have not yet been used for long enough to know. However our research has shown that immunity following natural infection remains for at least eight months and we have every expectation the vaccine immunity will be even longer than this.

If you are pregnant or could be pregnant please discuss this with your vaccinator.

Well at this stage there's not enough data about the effect of this vaccine on children. So it is currently recommended for those who are 16 and above further studies are underway to address this very question.

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