What You Need To Know About Covid-19 Testing Under Level 2
11th May 2020
Epidemiologist Simon Thornley says widespread serology testing for antibodies is necessary if New Zealand goes to Level 2
The Covid19 Plan B group has provided the public with vital information on how immunity is gained from exposure to Covid19, and on serology testing.
The most important information under Level 2 is not “cases” of people ill with the live virus, but the number of people who have had the virus and recovered – most without knowing it.
The current Covid-19 test only detects if the virus is currently in the body. Serology is a test of a person’s immune response to the virus, which 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.
“We can assess the spread of the virus with accuracy using antibody tests. The nation needs to know how widespread the virus is, so we can work out what precautions will work and are reasonable to ask of people.”
A serosurvey provided crucial information in New Zealand’s management of a swine flu epidemic in 2009, because it showed that the virus was much more widespread than initially thought, and therefore that the virus was much less deadly than initially believed.
Eighteen regions or countries have conducted serological surveys. The proportion of the population with positive Covid-19 antibodies ranged from between 0.5% in Colorado to 25.9% in Northern France. These findings indicate that the virus is too widespread to be countered with an elimination strategy. See Table 1 below.
What are antibodies?
Antibodies are ‘keys’ 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.
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.
What is the NZ government experience of antibody tests?
None have been rigorously tested in New Zealand yet. Government comments to media suggest that it wants local evidence of their accuracy, despite overseas studies showing excellent accuracy, with some now endorsed by regulatory agencies in the United States.
Does a positive antibody test prove immunity?
The long-term immunity associated with Covid-19 antibodies is not known. It is likely that they confer partial immunity, which has happened with antibodies for other 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.
Are there other types of immunity to Covid-19?
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 demonstrated that 83% of genetic test positive Covid-19 cases tested positive for T cells that react to the virus. 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.
Table 1. Prevalence of positive antibody tests to Covid-19 in surveys
|Region, Country||Sample size||Prevalence (%)|
|New York State, USA||15000||12.3%|
|Aspen Colorado, USA||198||9.9%|
|Los Angeles County, USA||NA||4.1%|
|Santa Clara, USA||3324||2.8%|
NA: not available.