Covid-19 testing in the era of omicron: everything you need to know

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Can you test positive for covid-19 without being contagious?

Even if you have received three doses of a coronavirus vaccine, a positive lateral flow test (LFT) result means you are contagious to others because viral proteins are present in large amounts in your nose or throat. To do this, the virus must be actively multiplying within your cells.

However, PCR tests continue to give positive results days to weeks after infection because they can detect trace amounts of viral genetic material that is not necessarily contagious.

How have the testing rules changed?

Since the surge caused by omicron variants began, many countries have reduced restrictions on covid-19 patients. In the UK, the quarantine period for infected people has been reduced from 10 days to seven days - as long as you get negative results in two LFTs, also known as rapid antigen tests. These must be completed on days six and seven, and at least 24 hours apart. However, people should be cautious about others and avoid those who are vulnerable.

Can the quarantine period be shortened any further?

In the U.S., the quarantine period has been shortened to five days for people who have no symptoms or whose symptoms are waning, but you should still wear a mask around other people for an additional five days. The UK Health and Safety Authority (UKHSA) said it had no plans to follow suit.

Why do some people still test positive on day seven even though they feel fine?

Symptoms may not reflect how much virus is present in a person's throat or nose - they may have replicating virus but no symptoms, or vice versa. "Viral loads vary greatly depending on the length of time you have been infected," said Al Edwards of the University of Reading in the UK.

The UKHSA estimates that between 10% and 30% of people are still positive on day 6 and 5% are still positive on day 10, although the guidance says you no longer need to isolate on day 11, no matter how you What the LFT results show.

What about confirmatory PCR testing?

In the UK, people who test positive for LFT will no longer need to take a follow-up PCR test, a temporary change that will come into effect on January 11. This is because background levels of covid-19 are so high - around 1 in 15 people were infected in the last week of December - that current positive LFT results are unlikely to be false.

Does this mean we can now rely solely on LFT?

Can't. In the UK, anyone with covid-19 symptoms but a negative LFT result must still take a PCR test to rule out infection because the false negative rate for LFT - which tells someone they really don't have covid-19 - is too high if You have symptoms, rely on them. Edwards said there are many reasons why LFTs are prone to false negatives, including people failing to swab their nose or throat properly and failing to adequately mix the swab with the test fluid. "LFT can only detect a large number of viruses."

Is omicron more prone to false negatives?

Opinions vary. UKHSA said preliminary investigations suggest LFT is as sensitive to omicron as it is to the delta variant that was dominant in most countries until December, although it is continuing to conduct further testing. But the FDA said LFT may be less sensitive in detecting omicrons.

LFTs that only involve swabbing the nose may be more likely to give false negative results for omicron, as some studies have shown that this variant is more likely to reach high levels in saliva and then in nasal mucus. A very small US study published online last week showed that in omicron infections, virus levels in saliva peak a day or two earlier than in nasal mucus, although the analysis only included five people.

Michael Mina of the US testing company eMed said: "Omicron may exacerbate this difference, and your throat and saliva samples will turn positive earlier." A study from South Africa also found that for omicron, saliva swabs are usually more positive than nasal swabs. The swab is more sensitive, while the delta variant is the opposite.

Should we use throat swabs instead of nasal swabs?

Opinions differ there too. In the UK, the LFT made by Innova directs users to take throat and nasal swabs, while others use only nasal swabs. In the United States, only nasal LFT is available, so people are less accustomed to taking throat swabs. "If possible, put the swab into the throat and into the nose," Dinan Pillay of University College London told a conference last week at Independent SAGE, a British group of independent scientists. "If the swab is shorter, stick your finger in More to reach the back of the throat.”

But Edwards said people should only use the test as directed. "If you change the way you use the test, it doesn't meet the manufacturer's claims. [Lower levels in the nose] probably won't have much of an impact on the accuracy of the test. Most people either won't get infected or they will Carrying a lot of viruses. We should insist on testing the right way.