Many countries now demand negative Covid-19 test results from incoming travellers.
But there is neither international agreement on the type of test nor the timing requirements. Even within the UK’s favourite foreign country, Spain, travellers are being given conflicting information on acceptable tests.
These are the key questions and answers.
What tests are there?
Covid tests look either for the presence of the virus in the body right now or evidence of a previous response to it by your immune system.
Those that look for current infection are broadly known as diagnostic tests. They can be molecular, such as PCR tests, which look for the virus’s genetic material; or antigen tests that detect specific proteins from the virus. Molecular tests are certainly preferred; the World Health Organisation says negative antigen diagnostic test results “should not remove a contact from quarantine requirements.”
An antibody test looks for antibodies created by your immune system in response to a virus. They can take several days or weeks to develop after you have an infection, and stay in your blood after you recovery. It is not clear if the presence of antibodies can give immunity to Covid-19 in future.
Which is the best Covid test?
Tests are never 100 per cent accurate, and vary in their “sensitivity” and “specificity”. Sensitivity means the proportion of carriers of the virus who are correctly identified; specificity refers to the proportion of non-carriers who are correctly identified. So 100 per cent sensitivity would mean no false negatives, while 100 per cent specificity means no false positives.
The most popular among government stipulating a particular test for travellers is the PCR (polymerase chain reaction) version. This is also the standard NHS test.
A swab is used for the back of the throat and/or the top of the nostrils. The sample is then processed to try to detect genetic material in the virus called RNA, which is evidence of the presence of Covid-19.
In a specialist laboratory, a solution known as a reagent is added to the sample. The resulting substance is then cooked: subjected to a series of alternating temperature steps using a “thermal cycler,” to create billions of copies of the RNA. This makes them detectable.
The analysis typically takes 12 hours. Since samples must be transported to the lab and are typically processed in bulk, getting a result can take much longer.
Are swab tests uncomfortable?
They are. Many people find the swab procedure