1.3 Screening programmes

During the COVID-19 pandemic, two main tests were used to detect the presence of the virus:

• PCR test, which detects segments of the virus genome
• lateral flow test, which detects the presence of virus antigens.

Both of these tests can detect the virus during a window of time when a person has recently been infected and is potentially infectious – typically this window lasts for about two weeks. In the UK the PCR tests became available early in 2020 and by May 2020 approximately 100,000 tests were being carried out daily. Lateral flow tests became available in late 2021.

Antibody testing can also be carried out in different ways, including:

• ELISA
• lateral flow tests, configured to detect anti-viral antibodies.

Antibodies can be detected from 1−2 weeks after infection, depending on the sensitivity of the test. This means that there is only a very short period of time when tests for virus and tests for antibodies will both give a positive result.

An important difference between the tests is that the lateral flow tests could be carried out at home, and they give a positive/negative result; the PCR test (virus) and ELISA (antibodies) are carried out by trained staff, usually in a laboratory, and they provide quantitative results.

It is important to note that testing for virus or anti-viral antibodies is carried out on different groups of people. The most reliable type of screening for assessing the prevalence of disease (virus) or immunity (antibodies) is carried out by random testing in the community.

For notifiable diseases such as measles, data from general practice and hospitals may give a reliable measure of disease incidence, because virtually all affected individuals have symptoms and will be identified. COVID-19 was a notifiable disease. However there was initially a huge amount of uncertainty as to what proportion of individuals were asymptomatic, and therefore might not have come forward for testing.

The advantage of random screening in the community is that it will identify people who are infected but who have no symptoms and do not know they are infected (virus +), or who had been infected in the past (antibody +).

The samples available in the ELISA: epidemiology laboratory represent a random screen from across the UK taken in August 2021. For the remainder of this week, you will be analysing these samples for the incidence of antibodies to the SARS-CoV2 spike protein, which are referred to as S-antibodies.