Explanation

SARS-CoV-2 nucleocapsid antibodies: what is measured?

SARS-CoV-2 is the coronavirus that causes COVID-19. The nucleocapsid protein (N protein) sits inside the virus, around its genetic material. After an infection, the immune system makes antibodies against this protein. The COVID vaccines used in the Netherlands only teach the body to recognize the spike protein on the outside of the virus and therefore do not produce antibodies against the N protein; this test can therefore show whether you have had COVID, even if you have been vaccinated.

What can it detect?

A past infection with the coronavirus, even if it caused no symptoms, separately from vaccination

Result

What does the result mean?

Positive result

A positive result means that antibodies against the N protein were found. This fits a past infection with the coronavirus; these antibodies do not develop after the COVID vaccines used in the Netherlands. The result does not say when the infection was, nor whether there were one or more infections. According to the LCI guideline (RIVM), antibodies say nothing about protection against a new infection or about being contagious. Because almost everyone in the Netherlands has had COVID by now, a positive result is the expected outcome.

Negative result

A negative result means that no antibodies against the N protein were found. This makes a past infection less likely, but does not rule it out: after a mild infection or an infection without symptoms, at an older age and with a weakened immune system, not everyone makes measurable antibodies, and the amount decreases over time. In addition, antibodies only become measurable 10 to 14 days after the start of symptoms. This result says nothing about antibodies after vaccination; the test for spike antibodies is meant for that.

After an abnormal result

A result of this test usually does not lead to further testing or treatment. With symptoms after a possible COVID infection, the GP assesses the symptoms themselves; the antibody test is not decisive for that. COVID-19 has no longer been notifiable since 1 July 2023.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM), antibodies can show whether someone has had COVID, but they give no certainty about protection, being contagious, or transmission. Because the vaccines used in the Netherlands do not produce antibodies against the N protein, the RIVM used these antibodies in the PIENTER-Corona study to also detect mild infections and infections without symptoms in vaccinated people. A positive result points fairly reliably to a past infection; a negative result rules one out less well.

What can affect the result?

Shortly after an infection, up to about 10 to 14 days after the start of symptoms, antibodies cannot be measured yet.
N antibodies can fall below the detection limit in the months to years after an infection, especially after a mild infection.
With a weakened immune system or medicines that suppress the immune system, antibodies are made less often.
After a vaccine made with a completely killed (inactivated) virus, such as some vaccines used outside Europe, N antibodies can also develop.

Who it is for

Who is the test useful for?

Useful for

Useful for anyone who wants to know whether they have had COVID, even after vaccination, for example if an infection was not tested at the time. Because almost everyone in the Netherlands has had COVID, a positive result rarely changes anything. With long-lasting symptoms, a positive result is no proof that the symptoms are caused by COVID, and a negative result does not rule that out. The test is not suitable for showing an infection that is present now, nor for seeing whether a COVID vaccination has produced antibodies.

How common is it?

According to the PIENTER-Corona study by the RIVM (Dutch National Institute for Public Health and the Environment), a little over 60 percent of the population had evidence of a past infection in spring 2022, and 97 percent in autumn 2023. In 2024, almost the entire population had had at least one infection.

RIVM, PIENTER-Corona study (2020 to 2024)

Drawbacks and risks

A negative result can wrongly give the impression that you have never had COVID, so that symptoms after an infection are explained differently.
A positive result can wrongly be read as protection against a new infection.

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