Covid-19 cases are rising quickly again due to the spread of Omicron variants BA.4 and BA.5.

According to Government data, 112,710 people reported testing positive in England and Wales in the seven days up to 25 June – a 35 per cent increase on the week prior.

However, the real numbers are likely to be far higher, as the Government is only tracking positive tests logged on its website, and the provision of free tests has ended.

The ZOE Covid study, which has been collecting data throughout the pandemic, says there are currently about 290,000 new infections across the UK every day.

It also predicts that about 3.26m people currently have symptomatic Covid-19.

Daily symptomatic infections have risen by 150 per cent this month, and according to ZOE data are at the highest level they have been for all but three weeks of the pandemic.

Meanwhile, some people have reported feeling Covid symptoms but are consistently testing negative.

Here’s what you need to know about test accuracy, and if you can still have the virus even if you are testing negative.

A negative result means it’s likely you are not infectious, however it is not definite.

Tests are not 100 per cent accurate, and you may have taken your test before the virus was detectable.

The Medicines and Healthcare products Regulatory Agency (MHRA) has stated “no test is 100 per cent reliable” because lateral flow tests cannot always detect low levels of the virus.

If you have only just developed symptoms but have tested negative, try waiting for a few hours or until the next day and testing again.

If you continue to test negative it is likely you have a similar virus that is not Covid.

The NHS lists the following as official Covid-19 symptoms:

A study published in the Clinical Epidemiology journal in October 2021, conducted by researchers from the University College London, Liverpool University, Harvard University, and the University of Bath, found that lateral flow tests are more than 80 per cent effective at identifying positive Covid-19 cases.

The study found the tests are more than 90 per cent effective at detecting the virus when someone is at their most infectious.

Another recent study combined results from seven studies (taking in more than 1,300 swab test results associated with time of disease onset) to create a model of the false negative rate for Covid over time since infection.

Their model suggested that in the first four days of infection (pre-symptomatic phase) the probability of a false negative in an infected person decreased from 100 per cent on day one (meaning a false negative was certain) to 67 per cent on day four.

It then decreased to 38 per cent on day five (the day of symptom onset) to a minimum of 20 per cent on day eight of infection (meaning one in five people still gave a false negative result despite having experienced three days of symptoms).

The false negative rate then increased from day nine (21 per cent) to day 21 (66 per cent).

UK-based GP Dr Gary Bartlett told The Independent: “What I often tell my patients is that lateral flow tests are really useful for asymptomatic screening (testing when you have no symptoms).

“If you have symptoms suggestive of Covid then you must get a PCR test, which are the gold standard test for Covid, as they are better at detecting Covid. If you have Covid symptoms, lateral flows are less reliable, as they can be associated with false negatives, often leading the person to believe that they don’t have Covid when in fact they do.”

He added that false positives – a test showing you have Covid when in fact you do not – are extremely rare, and only happen in “a fraction of a per cent of occasions”.

Professor Adam Finn, a member of the UK’s Joint Committee on Vaccination and Immunisation (JCVI) told the British Medical Journal: “If [lateral flow tests] come up positive that means you are potentially infectious to others and must self-isolate. They are not ‘green light’ tests: you cannot be sure that if the test is negative you are not infectious, and you must continue to take the usual precautions.”

To take an antigen test, you take a swab from inside your nostril, the back of your throat, or both, depending on the specific test you are using.

You then mix that swab with solution and drip it onto a device which contains a test strip.

The test strip has antibodies specific to the Covid-19 virus painted on it in a thin line.

These antibodies will bind to any antigen in the sample. If the testing strip detects the virus, it will show up as a thin red line next to the T on the device.

The Covid advice differs depending on where you live in the UK.

Self-isolation is no longer a legal requirement in England, but the NHS advises that people infected with Covid-19 “should stay at home and avoid contact with other people” to help reduce the spread of the virus.

In particular, you should avoid being in close contact with people at higher risk from coronavirus, for example if they are elderly or have a weakened immune system, even if they have had the vaccine.

Infected people should try to work from home if they can.

“If you are unable to work from home, talk to your employer about options available to you,” Government advice states.

Positive cases should try to stay at home and avoid contact with other people for five days after the day of their test.

Scotland’s national health information service also recommends self-isolation for people who test positive for coronavirus, but also advises that they book a free PCR test.

You should then isolate for 10 days from the date your symptoms started. If you have had a positive PCR or lateral flow test result but no symptoms, you should self-isolate for 10 days from the date of your test.

You may be able to end self-isolation early if you have two negative lateral flow test results in a row from day six onwards, taken 24 hours apart.

If you are a fully vaccinated close contact, for example someone who lives with an infected person, you can take daily lateral flow tests instead of self-isolating.

If you are a close contact who is not fully vaccinated, you should self-isolate for 10 days and book a PCR test.

The Welsh Government advises anyone who tests positive for Covid-19 to self-isolate. Anyone who experiences the main symptoms should also isolate and take a lateral flow test, which can still be ordered for free.

Positive cases should self-isolate for five full days, starting the day after their symptoms emerged or the day they had their test, whichever is earliest.

If you test negative on day five and six, you can then leave isolation. If you still have a high temperature or feel unwell, you should continue to self-isolate until it returns to normal, or you feel better.

If you test positive on day five or six, you should continue taking daily tests until you have two negative results in a row, taken a day apart, or until day 10, whichever is sooner.

The advice in Northern Ireland is to isolate if you have symptoms and book a free lateral flow test.

If you test positive, or are a close contact of someone who tests positive, you should isolate for 10 days from the date you took the test or the date your symptoms started, whichever is sooner.

You can end your isolation if you have two consecutive negative lateral flow test results taken 24 hours apart, the first of these taken no earlier than day five.

If your lateral flow tests continue to be positive after 10 days, you can leave isolation.

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