COVID-19 infection and immunity
When someone is likely to be infectious
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.
Length of the infectious period
If you get COVID-19, you are likely to be infectious from 48 hours before the onset of symptoms until 24 hours after your symptoms have ended.
Some people who have tested positive for COVID-19 can still be infectious for up to 10 days.
Sometimes people may have COVID-19, but not have any symptoms. These people may still be infectious.
Some people may test positive for COVID-19 after they have recovered and no longer have symptoms. They are unlikely to be infectious beyond 24 hours after their symptoms have ended.
Immunity and antibodies
When you get SARS-CoV2 — the virus that causes COVID-19 — your immune system kicks in and produces an immune response.
Your immune system starts creating proteins called antibodies. These antibodies fight the virus. Antibodies can remember what a virus looks like, which means that your immune system can spot and target the virus if you get it again. The immune system then destroys the virus.
This process is like what happens when your body encounters other common viruses, such as the flu.
While our body can produce an immune response to SARS-CoV-2, we do not know how long this immunity lasts. Initial research shows that antibody responses can last for several months in some people.
How vaccines work
Getting a COVID-19 vaccine produces the same immune response without you getting the virus. Vaccines give your body a head start so when you are exposed to the virus, your body already knows how to fight it.
Testing for antibodies
It is possible to test for antibodies in the blood if a person has had COVID-19 before. This is an antibody or serology test. Antibody testing may be helpful for finding out whether someone has had COVID-19 in the past but it is not helpful 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 only used in limited cases at the direction of a public health doctor. This is to support the investigation of possible new cases or to support patient management.
Antibodies in people who have had COVID-19
Antibodies attack 2 different parts of the SARS-CoV-2 virus. They are:
- the spike protein on the surface of the virus
- the shell of the virus, also known as the nucleocapsid.
Your body produces 3 different types of antibodies, also called immunoglobulins, to work against the 2 parts of the virus.
They are:
- IgM
- IgG
- IgA.
These 3 antibodies can be detected within 1 to 3 weeks after you have COVID-19.
IgM and IgG antibodies can be made at the same time. IgM and IgA antibodies decay more rapidly than IgG.
We know that:
- IgG antibodies made after having COVID-19 can be detected 2 or more weeks after symptoms began.
- Not everyone produces IgG antibodies when they get COVID-19. But they may still develop immunity.
- Studies show IgG antibodies can last for several months after you have COVID-19. But it is still unknown how long immunity may last.
As with other viral tests, antibody testing can have false-positive and false-negative results. These tests should only be used in appropriate situations and the results need to be interpreted carefully.
Reinfection with COVID-19
The latest evidence shows that getting reinfected with COVID-19 can happen within a short period of time. Reinfection will become more likely as new variants spread in the community.
If you get new COVID-19 symptoms and it has been more than 29 days since your previous infection, it is possible you have a new infection and should get a test.
It is unclear how common reinfection is with COVID-19. We are monitoring international and national data and will update guidance as new evidence is available.
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