Covid-19
Understanding the facts
More information about testing
How do the tests actually work?
PCR (Polymerase Chain Reaction) is the main test used to diagnose symptomatic individuals as per the NHS.UK. PCR is a technique used in molecular biology to analyse genetic material, hence to “amplify” small segments of genetic material from the virus, collected using a nasopharyngeal swab. During the Covid-19 pandemic proved to be an extremely useful tool to detect virus particles. This test tells you if you have Covid-19 at the time of testing, but not if you had it in the past. The technique is complicated and costly; not all are facilities are able to perform it and takes on average 4-6 hours to deliver a result. However, if a patient sample is being sent to a reference laboratory the results can take up to several days. Rapid PCR based tests have emerged, such as LAMP testing, which is mainly used to test healthcare workers. Follow link to animated video of the technique: https://www.sciencelearn.org.nz/resources/2347-what-is-pcr.
Steps:
-
Denaturation- the temperature of the sample is raised, and the strands separate.
-
Annealing – the temperature is lowered, and primers bind to the target DNA; this process is called polymerisation.
-
Extension - a heat resistant Taq DNA polymerase enzyme builds new copies using the original strand. The amplification process is exponential and with each cycle (20-30 in total) are created; approximately, 1 billion copies will result over a period of 2-3 hours.
PCR testing used in the UK have a sensitivity and specificity of greater than 95%
Limitations of PCR testing:
In general, the PCR technique has a few limitations, such as having lower specificity as primers can bind to nonspecific sequences on the template. After recovery, Covid-19 patients were still able to test positive for SARS-CoV-2 RNA due to serological viral load.
PCR Test
Lateral flow (for asymptomatic people)
Lateral flow testing is a rapid test for the viral proteins (antigens) found in the COVID-19 virus which works similarly to an at-home pregnancy test. LFT produces a result in 30 minutes and is intended for testing people who are not exhibiting symptoms (as PCR testing is preferred in symptomatic cases as it is more accurate). A respiratory (nose and throat) sample is taken which is placed on and travels along the test device; if there is sufficient antigen present it will bind to specific antibodies bound to a paper strip in a plastic casing causing a coloured line to form at a specific point (as well as a control line), indicating a positive result.
Lateral flow testing used (Innova LFT) has an estimated specificity of 99.68% which means a false positive rate of 0.32% . It has a sensitivity of 76.8% (for positive LFT confirmed positive with PCR testing) however this may be lower at 58% in asymptomatic people, and accuracy may be further reduced as self-testing is encouraged, and as such there is more human error.
Lateral Flow Test (For asymptomatic people)
How do COVID tests impact case rates?
Figure 1:To the left
The daily confirmed cases from February 2020 to June 2021.
By the end of April 2020, the COVID-19 epidemic in the UK showed signs of suppression and began to enter a slow decline- this was because the UK government had implemented stricter rules including lockdown and social distancing. A stark rise showed after August 2020 could be attributed to the increase in the number of testing centres increasing the capacity for mass testing and resulting in an overall rise in positive tests not necessarily signaling a rise in infection rates.
Figure 2: To the right
A stark increase of confirmed cases from 11th March 2020.
From 12th March 2020 the UK government was able to speed up the COVID testing procedure; prior to this a total number of 29,764 tests were performed in the UK. Matt Hancock announced on 24th March 2020 that the government had bought 3.5m antibody tests. The UK confirmed cases peaked on the 31st March 2020 by which point a total number of 143,186 tests had been done. This shows that by increasing the rate of COVID testing, more hidden cases were gradually confirmed. Since then, the government has also actively encouraged the UK citizens to take the COVID tests even when asymptomatic.
Figure 3: To the left
The data shows that in the number of confirmed cases was a marked increase between 28th August 2020 and 22nd November 2020, when a gradual decrease can be seen. This is due to the perfect storm of children going back to school, people returning from summer holiday and more people returning to workplaces. Moreover, with schools actively encouraged students to take the COVID tests, the demands for tests across the UK and confirmed cases soared from August to November.
Figure 4: the data below
This graph shows the number of virus tests performed in the UK in the last year. There is a general trend of increase in how many COVID-19 tests have been carried out. Although there was slight decrease in April, but this is because increasing vaccine rollouts led to less people taking the tests as they felt they were no longer at risk. (but should be cancelled )
