COVID-19: HOW GOOD ARE LATERAL FLOW TESTS WITH OMICRON? (Updated)

An LFT pack issued this week

We've planned for our Christmas family gatherings, and PM Johnson has given the go-ahead.  We’ll be relying on negative Lateral Flow Tests nearer the time to check whether we should get together.  How reliable are these LFTs?

Furthermore, the UK Government passed their ‘Plan B’ measures last week, two of which rely on negative Lateral Flow Tests:

• "Covid passes" to get into venues such as nightclubs and larger sports gatherings, being either vaccination status or negative LFTs (or proof of exemption)
• Anyone who's been in contact with a positive Covid case, and is fully vaccinated, now has to take lateral flow tests for seven days.

Again, how reliable are LFTs?  Are they good enough to support the Plan B measures?

As these questions have not been widely addressed, let's take a look.

 

FOR WHAT ARE THE LFTs DESIGNED?

The previous blogpost called “How Good Are Lateral Flow Tests?” mentioned limitations of LFTs. Specifically quoting the UK's MHRA as saying "lateral flow tests are only authorised to be used as a 'red light' test in order to find infectious people and ensure they self-isolate quickly, and not as a 'green light' for people who test negative to enjoy greater freedoms.”   

Yet that is exactly how we and the UK Government want to use them.

 

SO HOW EFFECTIVE ARE LFTs?

 All types of tests have two effectiveness measures:

1.  Sensitivity: How well they find positive cases. Conversely what proportion of tests will be ‘false negatives’.  For example, what if sensitivity is 80%?  That may sound high, but it would mean some 20% will be false negatives.  That's 1 in 5.
   
2.  Specificity: How often positive results are actually correct. Conversely not ‘false positives’

Sensitivity

As a positive test means the person self-isolates, we want a high sensitivity, and minimal false negatives.  Sensitivity falls below 100% for three reasons:

•  Lateral Flow Tests from different manufacturers vary in quality
• No LFTs pick up low levels of virus, as is the case in early or late stages of infection.  Especially early stages, as described below
  
• The test is only as good as how well people carry out the test, especially how well swabbing is done

The UK Health Security Agency (UKHSA) has just published their Technical Briefing 32 "SARS-CoV-2 variants of concern and variants under investigation in England".  In it they describe the initial testing of the various Lateral Flow Tests in use in the UK:

For the wet lab testing, "In summary, [all] the LFDs evaluated .....have detected the new Omicron variant".  Good news:

WHY ARE EXISTING LFDs WORKING?

This graphic from the FT shows the extent of mutations and their implications, without mentioning LFTs:

 

 

The FT goes on to say that fortunately  "Experts believe the UK tests are likely to continue to work because they detect a different protein from the much-mutated spike — and the variant’s potentially far higher viral load means they could even be more effective."

 

But...

WHAT ABOUT REAL-WORLD USAGE OF LFTs?

The UKHSA says:

The key issue is that LFTs are not as sensitive as PCR tests, and do not detect infections in their early stages.  People can be infected and become infectious before they take their next LFT:

In other words we cannot trust LFTs to detect infection with Omicron before we become infectious, and spread diseases amongst the people we meet.

This has been confirmed in the real world, such as at the Oxford Playhouse.  "Unfortunately, we are no longer able to go ahead with performances on Sunday 19 December (11am & 4pm), Tuesday 21 December (11am & 4pm) and Wednesday 22 December (11am & 4pm) of Robin Hood due to a positive Covid case within the company." The cast and crew have been taking both PCR and LFT tests every day.  Some negative LFTs have been accompanied by a positive PCR, which is more sensitive.

[Update 28/12/21 from the USA]  The CDC has just announced that their tests of LFTs with Omicron has preogressed from lab tests to real-world live cases.   "Early data suggests .... MAY have reduced sensitivity".  Not 'does have':

There is also some concern about brands of LFTs that only require a nose swab rather than also a throat swab.  Nose-only tests may not be as sensitive, and be prone to a higher level of false negatives.  Though it was one of the nose-only brands that showed positive for me on Boxing Day.

 

WHAT DOES THAT MEAN FOR US?

It is therefore vitally important:

(1) Not to regard a negative LFT result as not being infected, especially if have symptoms

(2) In any case to minimise close contact with others, especially indoors, to avoid catching the disease and avoid spreading it, even if everyone has recent negative LFTs

(3) To continue to adopt all other basic precautions, such as good ventilation, social distance and washing hands.  Especially when mixing with vulnerable people, such as older family members

WHAT DOES THAT MEAN FOR THE GOVERNMENT'S 'PLAN B'?

Last week MPs in Westminster debated and passed four proposals as elements of 'Plan B'.  The plan includes:

• The use of LFTs by the contacts of anyone confirmed infected with COVID-19
• The use of negative LFTs as an alternative to vaccination status as a condition of entry into many venues, such as nightclubs and sports stadia. 

Are LFTs good enough to be suitable for either purpose?

We know:

• Double jabs provide little protection against symptomaticinfection from Omicron, nor provide much protection against transmission.  Effectively now worthless for control of entry to venues
• Triple jabs with the booster only getting to around 75% protection
• As discussed above, LFTs do not identify Omicron (nor Delta) in the early stages of infection, and people can become infectious before their next LFT
  

So we shouldn't over-rely on LFTs, for any purpose, given false negatives are likely to be 1 in 5 or higher. Better than nothing, but there must be doubts about their use in Plan B.

Indeed is control of entry to venues as stipulated worthwhile?  Do we instead have to go back to closing venues and use lockdowns to try to reduce the speed Omicron is spreading?  Accompanied by suitable financial support from the Government.  Like it or frankly not!

The choice really should have been between:

• Double/triple jabbed AND negative LFT, or
• Close venues, despite the economic and societal consequences?

 

WHAT ABOUT AVAILABILITY OF LFTs?

LFTs can only work if they are readily available.  

The problem is that LFT kits are now in short supply.  Difficulties in distribution of such high numbers of them, even if they are in the country.

Deliveries to people's homes suspended nationally last week, and locally all three pharmacies were out of stock in recent days.  

What is someone supposed to do if cannot get necessary LFTs?  Legally, would they have to self-isolate?

Realistically we cannot rely on the availability of LFTs, on top of concerns about sensitivity.

 

WHERE DOES THAT LEAVE 'PLAN B' AND OUR FAMILY GATHERINGS?

LFTs are better than nothing, but do not really look good enough to be relied upon in the fight against Omicron.  Not adequate to self-test before visiting relatives at Christmas, nor suitable for Plan B measures 

That a significant number of Conservative MPs voted against the relatively mild 'Plan B' measures suggests they don’t believe the potential harms of Omicron.  They don’t see the risks of potential societal problems such as empty supermarket shelves. Yet good risk management is 'Hope for the best but prepare for the worst."

Is the risk of consequences such as empty shelves an over-reaction? Let’s hope that never happens.  The issue is that at every turn COVID-19 has proved to be at the worst end of expectations.  Risks of major adverse consequences, however low, surely cannot be ignored?

Indeed this is war.  It makes sense to reduce the spread of Omicron by whatever means we can.  Immediately. That means reducing the size and occurrence of groups of people indoors.  Not just Plan B, with ineffective LFTs, ineffective double vaccination, and boosters that are nowhere near perfect.  The Government needs to do more,. And quickly. Before society gets overrun by Omicron infection.

Of course this "Too little too late" Johnson Government has today (21st) taken no action.  Hang on for the ride, folks!