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: 13 September 2022

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When someone is likely to be infectious

Health care workers can assess whether a person is – or was – likely to be infectious by evaluating several factors:

  • the amount of virus in a positive swab test
  • when the person began experiencing symptoms (see case definition)
  • the severity of the person’s symptoms (more severe symptoms mean that a person is more likely to be infectious) 
  • when the person could have been exposed to someone else with COVID-19.

How infectious someone is may depend on a range of factors, such as how sick they are and how their immune system responds to the virus.

Read more about case definition and clinical testing for COVID-19.

Length of infectious period

People are considered likely to be infectious from 48 hours before the onset of symptoms. To prevent the spread of the virus people who have tested positive are required to isolate for full 7 days. Day 0 is from symptom onset, or day test taken, whichever came first.

Sometimes people may have the virus without any symptoms (asymptomatic cases); these people may still be infectious. Some people may test positive for COVID-19 after they have recovered and no longer have symptoms, but they are unlikely to be infectious beyond 24 hours after their symptoms have ended.

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

Once a person is infected with SARS-CoV-2 (the virus that causes COVID-19), their immune system kicks in and produces an immune response to the virus. This response is made up of different components, including proteins called antibodies. Antibodies are produced by the immune system to fight foreign organisms and substances that the body perceives as a threat, such as viruses. The antibodies can remember what the virus looks like, which means that they can quickly spot and target the virus if it is encountered again. The immune system then destroys the targeted virus. 

This process is similar to what happens when our body encounters other common viruses, such as influenza. While our body can produce an immune response to SARS-CoV-2 infection, it is not yet known how long this immunity lasts. Initial research has indicated that antibody responses can last for several months in some people.

Testing for antibodies

It is possible to assess if a person has been infected with SARS-CoV-2 in the past by testing for the presence of antibodies in the blood. This is known as an antibody or serology test. Antibody testing may be helpful for finding out whether someone has had COVID-19 in the past but is not useful for diagnosing new infections. This is because your body takes a few weeks to make antibodies after catching a virus.

Antibody testing is not available publicly on request in New Zealand. It 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.

Antibodies in people who have had COVID-19

Antibody responses can be directed against different parts of the SARS-CoV-2 virus, such as the spike (S) protein on the surface of the virus or the nucleocapsid (N) protein that forms the shell of the virus. There are different types of antibodies, also called immunoglobins (Ig), that the body produces in response to infection. IgM, IgG, and IgA are three types of immunoglobins that are produced against the S and N proteins and can be detected within 1-3 weeks after SARS-CoV-2 infection. IgM and IgG antibodies can arise simultaneously, however, IgM (and IgA) antibodies decay more rapidly than IgG.

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

  • IgG antibodies produced in response to SARS-CoV-2 infection 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 IgG, but may still develop immunity
  • studies have shown that IgG can last for several months in previously infected people, but it is still unknown how long immunity to SARS-CoV-2 may last.

Similar to viral tests, antibody tests checking for immunity can also have false-positive and false-negative results. They should be used in appropriate situations and the results need to be interpreted carefully. For example, the small number of people who have had COVID-19 but didn't produce IgG might have a false negative result after being tested for these antibodies, so another test might be required for these individuals.

Read more about assessment and testing for COVID-19.

How vaccines work

The COVID-19 vaccine stimulates the body’s immune system to produce antibodies against the spike protein of the SARS-CoV-2 virus, so your body can quickly spot and attack the virus if you are exposed to it in the future.

Read more about COVID-19 vaccines.

Reinfection with COVID-19

The latest evidence shows that getting reinfected with COVID-19 can occur within a short period of time. Reinfection will become more likely as new variants spread among the community.

If a person develops new symptoms consistent with COVID-19, and it is 29 days or more since a previous infection, it is possible they have a reinfection with COVID-19 and they should test using a rapid antigen test.

It is unclear how common it is to be reinfected with COVID-19. We are constantly monitoring international and national data and updating our reinfection guidance as new evidence becomes available.

Read more: Getting reinfected with COVID-19

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