COVID-19: WHAT DOES BEING ASYMPTOMATIC MEAN?

In the context of COVID-19, to be asymptomatic means someone has tested positive for the viral RNA or the antibodies, but has not displayed any of the classic initial symptoms.  No temperature, no cough, no loss of taste nor loss of sense of smell.

Reports are of over half of positive "Got it" tests are for people who are asymptomatic.  The ONS (Office for National Statistics) suggests as many as 72%.


But what does being asymptomatic mean?

1. Why would that happen?
2. Are they infectious?
3. Should they be included in estimates of new daily cases?
4. How many people are infectious in total?
5. Could they suffer from LongCOVID?

Firstly we have to realise that, assuming a test has a good 'specificity' such that a positive test is reliably positive, which is usually the case:

• A "Got it" test is testing for viral RNA, usually where a swab is taken from the nose and/or throat.  This could be remnants of inactivated viruses, and not necessarily active infectious viruses.  As it says in this Lancet article, the common method "PCR does not distinguish between infectious virus and non-infectious nucleic acid"
• An antibody test, using a blood sample, is confirmation of a previous infection with COVID-19

Ideally anyone being tested should have both types of tests.


WHY WOULD SOMEONE BE ASYMPTOMATIC?

Fundamentally someone would be asymptomatic if the body's immune system fights off the virus before infection is bad enough to produce symptoms.  That could be for one of several reasons:

• The dose of virus is low enough for the body to respond before infection takes hold, but high enough to be picked up in a test.
• The person has had COVID-19 before, whether or not they have had symptoms, and there is enough immunity to tackle the virus as soon as it appears.  This could be infection from a different strain of the virus, contracted somewhere different in the world, such as in this example
• Cross-immunity from having had another corona virus, probably one of the four types of common cold viruses.

In each case the positive "Got it test" may be for one of two reasons:

• The person has only just been infected, the viruses are active, and the person is probably infectious 
• The person only has virus remnants after the immune system has been successful.  This can be some time after symptoms have disappeared, if someone coughs up viral remnants from the lungs into the area being swabbed.

It's important to remember that by reducing the amount of dose received from an infectious person, it is possible to reduce the severity of symptoms, and potentially avoid symptoms altogether.  Whether that is by keeping some extra distance from others, the small advantage of wearing a surgical mask, and/or good hand hygiene.


IS AN ASYMPTOMATIC PERSON INFECTIOUS?

As indicated above:

• Probably yes if someone has just been infected
• No if the immune system has defeated the virus

Without knowing when they were infected, we can't tell.  For tests across a population, we can only guess what proportion is infectious.


SHOULD ASYMPTOMATIC RESULTS BE INCLUDED IN THE COUNT OF NEW CASES?

There are three reasons why asymptomatic results should be included:

• We need to know the extent to which the infection is spreading
• They could be infectious
• It is possible they could suffer from LongCOVID, so that possibility needs to be assessed

HOW DO WE ESTIMATE THE TOTAL NUMBER OF PEOPLE INFECTIOUS?

The relationship between positive "Got it tests" and actually being infectious has been studied a little, by culturing live SARS-CoV-2 virus from human samples.  A review of 14 such studies showed data was "sparse on how the PCR results relate to viral culture results".

So we need to estimate.  If we know the total daily new cases, there are two components:

• People who become symptomatic, and are definitely infectious for a while
• People who remain asymptomatic, and may be infectious for a while

For those who become symptomatic, it typically takes five days for someone to display symptoms after becoming infected with COVID-19 (in the range 2-14 days).  So if we assume most people then self-isolate, for every daily new case at least five people will be out and about unaware they have the virus.  Studies suggest that people are more infectious before they have symptoms.

For those who remain asymptomatic, but have enough virus to test positive, again they might have enough virus to be infectious for at least 5 days.  Though could be considerably longer.


With at least half of positive tests being for asymptomatic cases, that means that overall the number infectious is at least 5 times the daily new cases.

According to the latest data analysis of the ONS data, they have been using a ratio of around 8 for the number of people infected, but not necessarily infectious, with 10.3 in the most recent report.

So it is reasonable to regard the number infectious in any area to be around 5 times the new daily cases.


The total of 'New Confirmed Cases' for England is currently around 1000 per day.  We've assessed that the true number of daily cases is also around 5 times that value, to include everyone who hasn't been tested.  That means a daily rate of some 5000 total, and some 25,000 people across England are infectious but living life as normal.   Multiplying the 'confirmed new cases' by 25 overall.


SWINDON AS AN EXAMPLE

Swindon is currently an area of concern on the Public Health England coronavirus watchlist, with the highest daily infection count for its population in the south west.

Swindon has a population of some 222,000 people.  The New Confirmed Cases a few days ago was 23, which we can use for illustration.




This is some 10 per day per hundred thousand, but needs to be multiplied by 5 to get a truer estimate of daily infections, which would be 50 infections per day per hundred thousand.  If that rate was steady every day, the number infectious would be found by multiplying again by 5 and adjusting for population:

• A town like Swindon could have 575 people infectious out working, shopping, or some other activity
• A city like Oxford, with around 150,000 people, could have 375.  
• A town like Watford with around 100,000 people, could have 250

The problem is we just don't know who.  Neither do they.

I don't want to catch COVID-19 because of the six risks. That's not just death, with LongCOVID coming through as a bigger problem in terms of the severity of effect and the number of people suffering.

Walking out of my home with that sort of number infectious, is not appealing.  My simple rule at present is outdoors, not indoors.  The one exception being to buy food. 


A MORE COMFORTABLE LEVEL OF INFECTION
 

At the beginning of May Professor Chris Whitty, Chief Medical Officer for England, suggested a sensible maximum rate of infection would be 1000 in total for the country.  That's equivalent to about 2 new cases a day per hundred thousand, and would mean if that was taken as a maximum in an area:

• A town like Swindon would have a maximum of 22 people out and about infectious
• Maximum 15 in Oxford
• Maximum 10 in Watford

Provided I took sensible precautions, that lower level of infections would mean I would be much more comfortable about going inside premises, whether that was a shop, a pub, a restaurant or theatre.  We need to get down to that sort of level to open the economy properly, keep schools open, and live life near-normally again.


COULD ASYMPTOMATIC PEOPLE SUFFER FROM LONGCOVID?

The research on LongCOVID is in its infancy, with little information available.  In principle:

• Lethargy: Anyone who has been fighting off an infection, even without symptoms, could suffer from ongoing lethargy
• Organ damage: If the virus got into the blood stream, other organs could have been damaged and result in ongoing symptoms

The two risks are likely to be low, but not zero.  Until further research results are available, we should assume that some of those who have been asymptomatic will suffer from LongCOVID.