What you need to know about Covid19 serology

By Simon Thornley

19 May 2020

Why does New Zealand need a serosurvey?

New Zealand urgently needs to test for antibodies to Covid-19. The standard test for Covid-19 at present is a genetic test that only detects whether or not the virus is currently in the body. Serology is a test of a person’s immune response to the virus and persists long after the virus has disappeared. This test gives important information about who in the community has recovered from infection and is thus unlikely to get the infection and pass it on to others. The overwhelming picture from this information is that the virus is much more widespread than is shown from genetic test positive cases.

In the response to swine flu in 2009, a serosurvey provided crucial information to dial back the clamour to stamp out the virus, since that survey showed that the virus was much more widespread than initially thought. As a consequence, it also followed that the virus was much less deadly than initially believed.

We can now count a total of 18 regions or countries that have conducted serological surveys and reported results in English to determine the extent of population exposure to the virus (Table). The proportion of the population who were found to have positive Covid-19 antibodies ranged from between 0.5%  in Colorado to 25.9% in Northern France. In some studies, the rate of positivity increased substantially as the study progressed. Even in the low prevalence regions, these findings strongly indicate that the virus is widespread and unlikely to be amenable to an elimination strategy.

Table 1. Prevalence of positive antibody tests to Covid-19 in surveys from around the world.

Region, Country Sample size Prevalence (%)
Northern France 171 25.9%
Guilan, Iran 552 22.0%
Gangelt, Germany NA 14.0%
New York State, USA 15000 12.3%
Barcelona, Spain 578 11.2%
Wuhan, China 1402 10.0%
Aspen Colorado, USA 198 9.9%
Miami-Dade, USA 1400 6.0%
Switzerland, Geneva 760 5.5%
Los Angeles County, USA NA 4.1%
Finland 147 3.4%
Kobe, Japan 1000 3.3%
Moscow, Russia 1000 3.0%
Santa Clara, USA 3324 2.8%
Netherlands 7361 2.7%
Denmark 9496 1.7%
Colorado, USA 5455 0.5%

NA: not available.

What are antibodies?

Antibodies are like keys in a lock that the body makes in response to viruses and other bacteria. Antibodies only fit a specific virus or bacteria. The shape of the antibody locks on to the microbe so that the body’s immune system can more easily eliminate the virus. Once a high proportion of the community have antibodies to the virus, it becomes very difficult for the virus to spread throughout the community, since it is hard for the virus to find new susceptible people to spread to.

Antibody tests are generally not used to diagnose the infection, since there may be a delay of one to three weeks from the time of infectiousness with the virus to the time that antibodies are generated by the body. Genetic tests, such as PCR, are usually used for making the diagnosis as they are positive earlier in the course of the illness.

What sort of antibody tests are available for Covid-19?

An antibody test generally involves the collection of venous blood or a finger prick to collect capillary blood. A number of test kits have been authorised by the US Food and Drug Administration for use for Covid-19. The Center for Disease Control has developed a test which is reliable for detecting SARS-CoV-2. The test is claimed by the organisation to be 99% sensitive (low false-negative rate) and 96% specific (low false-positive rate).

At present, tests of immunity are mainly recommended for assessing the extent of infection, and what proportion of the population has had mild disease from the virus. Until more information comes to light, researchers are not certain that test-positive individuals are unable to be re-infected, although this is likely to be true.

Are there other types of immunity to Covid-19 apart from antibodies?

As well as using antibodies which come from “B” white blood cells, our immune system also has “T” cells that recognise the virus directly, without the need for antibodies. A recent study from Germany has demonstrated that 83% of genetic test positive Covid-19 cases tested positive for T cells that react to the virus. Also, 34% of healthy blood donors, who were test-negative for antibodies, had evidence of reactive T cells, but at lower levels than cases. It is likely that these T cells confer some immunity to the virus, but it is unclear to what extent such people are protected.

What is the NZ government experience of antibody tests?

A wide range are available, but none have been rigorously tested in New Zealand yet. In order to be confident that these tests are useful, media interviews suggest that the government requires local evidence of testing their accuracy, despite overseas studies showing excellent accuracy with some tests. A number of tests are now endorsed by regulatory agencies in the United States.

If someone tests positive for antibodies, does that prove immunity?

The long term immunity associated with Covid-19 antibodies is not known. It is likely that they confer partial immunity, as seen with other antibodies for coronaviruses. This depends on the dose and route of administration. For example, in an animal study, mice administered coronavirus in the nose maintained immunity for at least 12 months, however, those that had exposure to the virus by mouth had high levels of immunity at one months, but lower levels at 6 and 12 months.


New Zealand urgently needs to test for antibodies to determine community exposure to the virus. If antibody levels are high, then this suggests that the virus is widespread. This also means that the virus is much less deadly than we feared.