CHRISTMAS has been given the green-light by Boris after he said there will be no new restrictions before the big day.

The PM urged people to take a Covid lateral flow test before seeing family, especially the vulnerable, however.

If you get a positive test, you need to self-isolate immediately and follow up with a PCR test because it is highly likely you have Covid.

A negative test – a singular line on the C mask of the pregnancy-style kit – is what everyone holds their breath for.

If you are negative (hooray!), it means you are free to carry on as normal and see loved ones, as guidance says.

But how much can you rely on the negative result to safely hug your Grandma? Is it still possible you can get a negative test on Christmas eve, but still infect your family over festivities?

Can you test negative on lateral flow tests and still infect your family?

The short answer is yes, you can test negative on lateral flow tests and still infect your family.

But there are a number of caveats.

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A graph produced by Harvard Professor Dr Michael Mina, published in the New England Journal of Medicine, gives a simple illustration of how lateral flow tests work during Covid infection.

It shows that lateral flow tests give a positive result during the infectious window of a person’s Covid disease, which is useful.

But, unlike a PCR Covid test, it cannot always tell you if you are in the early stages of the disease – the pre-infectious period.

You are likely to get negative results in the first couple of days after exposure to the virus.

But a positive result could be mere hours away, when you are then infectious and unknowingly spreading it to others.

This highlights the importance of using lateral flow tests daily when there is potential you have been exposed to Covid.

If you have symptoms 

If you get a negative lateral flow test result, but have symptoms of Covid, it’s risky to trust the kit.

Dr Nathan, an A&E doctor in London, who has recently been producing helpful posts on his Instagram page (@expedition_doctor), told The Sun: “If you have symptoms, you should isolate immediately and book a PCR test, even if you have a negative LFT. 

“This is because a person with symptoms may have a negative LFT even when they actually do have Covid. 

“Since the beginning of the pandemic, the guidance has always been to self isolate and book a PCR if you develop symptoms.”

If you don’t have symptoms

If you don’t have symptoms of Covid, and get a negative lateral flow test, it removes the odds that you can pass the virus on to your family.

However, as described above, you could just be in the pre-infectious period.

The NHS says: “A negative result means it’s likely you are not infectious.

“But a negative test is not a guarantee you do not have Covid-19 and there’s still a chance you may be infectious.”

Lateral flow tests are also not entirely accurate when it comes to detecting the coronavirus, even if you are contagious. 

A major review found lateral flow tests missed 60 per cent of positive cases that would have been found through a PCR, meaning many people with Covid are given what’s called a “false negative”.

If you are coming out of self-isolation

Many people will be coming out of self-isolation just in time for Christmas, having caught Covid – possibly Omicron.

Guidance says after seven days from your symptoms or positive test (reduced from 10 under new rules), you are able to come out of isolation, as long as you have a negative lateral flow test on day six, and 24 hours after.

But many will be wary that they are still carrying the virus, and risk passing it on to someone in their family. 

Dr Nathan said lateral flow tests are not likely to stay positive after the infectious period is over.

The tests typically only give a positive result during your infectious window – the first few days of your symptoms. 

Outside of this, it is unlikely you are contagious to others. This does not mean there is zero risk, however.

Dr Nathan said: “LFTs are less likely to stay positive after the infectious period is over, and they can continue to be used as recommended.

“PCR tests can stay positive up to 90 days after having Covid, and you should not take one unless you develop symptoms again, you have a positive LFT, or you need one for travel.

"Any positive PCR regardless of the 90 days guidance means you should complete 10 days self isolation.”

Paul Hunter, professor of medicine at the University of East Anglia, told MailOnline when discussing new self-isolation rules, said: “People are most infectious in the first five days, after which time infectiousness falls.

“Some people are no longer infectious after three days and it makes no sense to keep them locked up.”

When should you take a lateral flow test?

Experts advise taking a lateral flow just before seeing your nearest and dearest – not the night before, or hours before.

The closer you take the test to socialising, the more confident you can be that you are free of the virus.

An expert has previously shown how fast the result of a lateral flow test can change from negative to positive, amid the rapid spread of Omicron.

Should you see your family?

If you have recently had Covid, and your isolation ends just as Christmas starts, it’s worth taking a look at the government guidance.

It says that, “you should be cautious, even with negative lateral flows”. 

“Those who leave self-isolation on or after day seven are strongly advised to limit close contact with other people in crowded or poorly ventilated spaces, work from home and minimise contact with anyone who is at higher risk of severe illness if infected with Covid-19,” it says,

The NHS says: “If you still feel unwell after a negative test, stay at home until you're feeling better. Contact a GP if your symptoms get worse or do not go away.

“If you're being sick, have diarrhoea or have a high temperature, stay at home until 48 hours after they've stopped.

“If you get Covid-19 symptoms after the test, you need to get tested again.”

With so many things to consider, ultimately, people make their own decisions based on their own circumstances.

Dr Hayley Jones, senior lecturer at the Bristol Medical School clinical epidemiology unit, said an individual’s decision will depend heavily on context. 

She told The Telegraph: “A negative LFT result, especially if immediately before the contact, means you’re less likely to be infectious at that point in time.

“Your decision on whether this reassures you will depend on a number of other factors, such as whether your elderly relatives are vaccinated, whether they are particularly vulnerable, your own recent levels of social contact (particularly any known contact with someone with Covid) and, obviously, whether you have any symptoms.

“We talk of pre-test and post-test likelihood of having Covid.

“If you think there’s a high chance that you are infected, then you should not let a negative LFT alone change your mind, and consider a PCR.”

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