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Scientist Explains How Your Lateral Flow Test Changes According To How Infectious You Are

Written by on January 5, 2022

A woman at home is using a lateral flow testing kit.

Lateral flow tests are accurate in tracking how infectious someone is, according to an epidemiologist.

Prime minister Boris Johnson has announced from the January 11 the rules around PCR tests and Lateral Flow tests (LFTs) will be changing.

Asymptomatic people who test positive on LFTs no longer need to take follow-up PCR tests. Instead, they have to start their self-isolation period immediately, rather than waiting for another test result.

Symptomatic people are advised to continue taking PCRs.

LFTs are an increasingly important part of the UK’s response to Covid even though they have been criticised in the past for not consistently detecting the virus.

US epidemiologist and immunologist Dr Michael Mina explained how they work on Twitter amid concerns LFTs were not going to pick up the new highly-transmissible variant, Omicron.

Responding to questions about why some people stay negative on LFTs the first few days they have symptoms, he explained: “This is expected. Symptoms don’t [equal] contagious virus.

“This is literally a reflection of the fact that vaccines are doing their job!”

 He tweeted again: “Rapid tests work with Omicron.

“Omicron is mutated mostly in spike [protein].

“Rapid tests don’t detect the spike.”

Dr Mina added that newly symptomatic people who are coming up negative on LFTs are “probably not infectious” but it’s advisable to test again the next day.

“I am primarily trying here to explain that it is expected for people to show symptoms earlier than before, as a result of vaccination and developed of acquired immunity.

“My point was that symptoms SHOULD be expected to arise early now that people’s immune systems don’t have to spend 5+ days realising someone is infected with a new virus. Now our immunity kicks in fast, and with it, symptoms do too.”

He also shared a series of photos of LFTs each day during someone’s isolation period.

These images clearly show the positive line on each test strengthening during the first part of the self-isolation before fading out again by day 10.

This demonstrates that the individual who was taking the tests was at their most infectious during the middle of their isolation.

Another graph from the BBC was also circulating online, emphasising how LFTs pick up high levels of infection compared to PCRs.


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