One (of the many) things that irritates me is when someone says a new variant is “more transmissible”. Why does it irritate me? Well, it’s an incorrect term. Transmissible means “being able to be spread from one person to another”, well we know SARS CoV2 is transmissible… but it cannot be “more transmissible”, it is either transmissible or it is not. More transmissible would mean that it was better able to “move” from one person to another and we have no evidence of that at all… it hasn’t suddenly sprouted wings!
So, what types of changes to SARS CoV2 might give it a fitness advantage? (Note: this is all theoretical… no one actually knows what the “fitness advantages” in SARS CoV2 are!)
Increased replication rate inside the host
If SARS CoV2 becomes better at replicating inside human cells such that it takes less time to get up to sufficient numbers of virus particles to cause symptoms to develop e.g. cough, then the time from exposure to infectiousness will be reduced. If this also corresponded to lots of virus in the respiratory secretions, then the time from someone being exposed to the virus to when they are able to spread it (the incubation period) would be much shorter.
This would provide SARS CoV2 with a “fitness advantage” as people could spread the virus to others before they even realised they were infectious. When you factor in that tests for the virus such as PCR can take about 48 hours to give a result you have a situation where the virus can spread before the test result is back to warn people that they are infectious.
This fitness advantage would lead to faster spread of SARS CoV2 within the population, and make the virus spread more efficiently.
Easier entry of virus into cells
Every infection has an infectious dose; that is the number of infectious particles that are required to cause an infection. Part of this infectious dose has to do with how easily the microorganism enters its host’s cells. If a microorganism enters the cell easily then the infectious dose will be low, and the fewer the number of particles required to cause the infection. Infections with low infectious doses spread more easily.
For example, Norovirus is notorious for causing massive outbreaks in institutions like hospitals, care homes and nursery schools. This is because every gram of stool from a person with Norovirus gastroenteritis has approximately 100 million infectious doses of virus; it doesn’t take much to allow Norovirus to spread.
We don’t actually know the infectious dose of SARS CoV2, but if a variant comes along that gets into cells better, and hence has a lower infectious dose, then this can be expected to give a “fitness advantage” that allows the variant to spread more efficiently, allowing cases of this variant to increase.
Increased survival in the environment
Viruses like SARS CoV2 spread via droplets, both directly if someone coughs in your face (yuck!) but also if someone coughs into their hands and then doesn’t clean their hands before touching a surface. If you come along and touch that surface, you pick up the virus and then might rub that around your mouth or nose thereby infecting yourself.
Any microorganism capable of surviving for long periods of time in the environment in an infectious state will have more time to potentially come into contact with a new host.
In the case of SARS CoV2, perhaps a new variant has a more stable envelope which protects it from desiccation (drying), allowing it to last for many days on a surface. This “fitness advantage” will give more time for the contaminated surface to be a reservoir of new virus for new hosts and help the variant spread more efficiently.
Reduced immune activation
Most of the symptoms of Covid-19 are related to the immune response of the host. These symptoms tell the host they are infected and therefore might be a risk to others. This is why anyone who feels unwell with any infection is told to stay at home and don’t share their germs!
But what would happen if a new variant of SARS CoV2 came along that caused less immune activation and therefore fewer symptoms? In this case the new host wouldn’t know they were infectious and therefore may not stay at home. This would give the new variant a “fitness advantage” in that there would be more opportunities to spread from an asymptomatic person to others without them even knowing… a stealth infection! This is what is known as asymptomatic transmission.
This type of “fitness advantage” would only be picked up when people without obvious exposure to infected people suddenly developed symptoms themselves or had a positive test even if they themselves also had no symptoms. This is what happened over Christmas when the Government encouraged lots of random tests of asymptomatic people (before they went to work, school, the pub or their Christmas party). In this situation you will “find” thousands of asymptomatic infected people and cases will spike yet “illnesses” will not.
It artificially causes a “wave” of concern when one isn’t really there! In December 2021 there was an average of 30,000 - 40,000 cases a day (23rd July - 12th Dec) until the “take a test advice” was issued over the Christmas period when there was an average of 180,000 cases per day but “illnesses” had hardly increased and now the case numbers are back down (48,000 17th Feb 2022). Who cares if people have an infection if there is no ill-effect? How many of us have athletes foot (about 15% of the UK population, that’s 9,000,000 people) at any one time or Herpes Simplex Virus (HSV is a very common infection with as many as 7 in 10 people, or 70% of the UK population, that’s 42,000,000 people) or Hepatitis C Virus effecting 200,000 people in the UK, half of whom are undiagnosed! We live on a Planet with many more viruses, fungi and bacteria than humans; we cannot eliminate all these species and life forms, we MUST co-exist with them!
Our immune system has a special ability to recognise pathogens it has seen before; this is the basis of the adaptive immune system and our antibody responses to infections. Vaccines also help prime this adaptive response.
If a variant of a virus comes along that is less well recognised by our previous immunological memory, and our antibodies are less reactive to the new variant, then the variant will have more time to replicate, make us sick, and spread, before the immune system catches up and gets rid of the new variant. This is what happens a lot with viruses such as Influenza Virus; it changes a lot over time making our immune system out of date rapidly, and hence we need yearly vaccines against the new variants of Influenza Virus.
If a new variant of SARS CoV2 comes along that is able to get around our current levels of infection or vaccine derived immunity, then it will have a “fitness advantage” and will have a better opportunity to spread to other people before our immune systems catch up.
It’s all about fitness!
So, just to reiterate, we have no idea what the “fitness advantage” of any new variant of SARS CoV2 might be. It could one of the points above or it may be none of them… we just don’t know. However the fact the virus moves or travels is “transmission”; it travels from one person to another. Transmission is the route e.g. coughing. Fitness is the efficiency of spread NOT transmission. This efficiency is Darwin’s fitness of the species; the virus “upgrades” from a bus to an aeroplane as a more efficient mode of “travelling” to the next person. Heck! It could just be that it is winter and ALL coronaviruses spread better in winter because of changes in humidity, droplet size, close proximity of people indoors and snogging under the mistletoe.
But, whatever the reason for us seeing more cases of Covid-19 at the moment, let’s please try and keep our terminology correct so we can make rational decisions about how to control the spread of any new variants of the virus… like wash your hands, stay at home if you’re ill, cough into a tissue and then wash your hands again! (I KNOW, I sound like a broken record!)
Viruses do not sprout wings, they “move” from host to host (they are transmissible) but it is the “fitter ones” that move better. If you “have to use” the term “more transmissible” can you at least do it out of my earshot… at least for the sake of ECIC who has to listen to all of my grumbles :-)