COVID-19: What we know about infection and immunity

The latest information about when people with COVID-19 are likely to be infectious, and their immunity afterwards.

Last updated: 28 September 2020

On this page:

Download this information, along with information on testing for COVID-19, in a factsheet: 

How to tell if someone is infectious

Healthcare workers can work out whether a person is - or was - infectious from a positive test for COVID-19 combined with:

  • any virus detected in the test
  • the person’s symptoms (more severe symptoms mean that a person is more likely to be infectious) and when their symptoms started, particularly coughing, and 
  • whether people they had contact with became ill with COVID-19 later.

It is not possible to say exactly how long someone will be infectious when they have COVID-19. Studies so far show that many people could be infectious for seven days after symptoms start, and some severely-affected people may be infectious for longer. This is still under investigation because this is such a new disease.

It all depends on how severely they are infected and how their body's immune system responds. 

Sometimes people may have the virus without any symptoms (asymptomatic cases); these people may also be infectious.

Some people may test positive for COVID-19 for some time after they have recovered and no longer have symptoms, but they are unlikely to be infectious beyond 72 hours after their symptoms have ended.

Immunity and antibodies – what we know so far for COVID-19

Once a person is infected with COVID-19, their body will produce cells (antibodies) that ‘remember’ the virus. 

For other common viruses, such as the flu, when our body produces antibodies and ‘remembers’ a virus we are then immune from catching the same virus again. While we can produce antibodies in response to COVID-19 infection, we do not know yet how long immunity lasts.

Testing for antibodies

Antibodies are proteins produced to fight viruses (and other organisms and substances that the body perceives as a threat, such as bacteria). This response is called an immune response and is one of the ways the body protects itself.

By measuring the number of these antibody protein cells, it is possible to find out if a person was infected in the past. This is known as an antibody or serology test. This test looks for the body’s response to the virus, not the virus itself. 

The antibody test may be helpful for finding out who has had COVID-19 in the past but is not useful for diagnosing new infections, because it takes two to three weeks after catching a virus for your body to make the antibodies that this type of test detects.

This sort of test is not available publicly on request in New Zealand. This type of test is currently only used in limited cases, as directed by a public health doctor, to support investigation of possible new cases or to support patient management

Types of immunoglobin

Immunoglobin is a type of protein that acts as an antibody. IgM, IgG and IgA are types of immunoglobin that the body produces as antibodies in response to the virus that causes COVID-19. 

The body produces IgM antibodies first when it detects and responds to an infection. Both IgM and IgG are found circulating in the blood. IgA can also be detected in the blood but is produced in the linings of the respiratory tract (inside the nose, mouth throat and lungs), in the digestive system, and in saliva and tears. IgG is the antibody that can potentially tell us if someone is immune to COVID-19, but it can’t tell us how long the immunity will last or how effective the immunity would be against different strains of the virus in future (known as ‘immune cover’).

Antibodies in people who have had COVID-19

What we know so far about antibodies in people who have had COVID-19:

  • IgG antibodies produced in response to catching COVID-19 are most reliably detected two weeks or longer after symptoms begin
  • not everyone who gets infected produces IgG antibodies- a small proportion of people do not produce these, but even without these antibodies, some may still be immune
  • some patients appear to only produce IgA antibodies
  • nobody knows how long immunity to COVID-19 lasts.  

Like viral tests, antibody tests checking for immunity can also have false-positive and false-negative results. For example, people with COVID-19 who didn't produce IgG might have a false-negative result afterwards, if they are tested for these antibodies.

Achieving herd immunity

When enough people in a population are immune (through vaccination or previous infection), a virus cannot spread as easily in the community. This is known as herd immunity, or herd protection.

The world is currently at the beginning of the pandemic and the situation is evolving. Nobody knows how long immunity lasts after someone has had COVID-19, and vaccines are still under development. 

This means herd immunity has not yet been achieved in any country. 

Back to top