The simple answer, despite what the Government and the media say, is “no one knows”. But everyone says Delta is more transmissible. Surely, they wouldn’t say this if it wasn’t true!?
Transmission is the spread within a population. To be more transmissible something would have to spread to more people under the same conditions.
Okay, let’s ask the question “how would you work out if the Delta variant is more transmissible than the original SARS CoV2?”
To do this you would have to compare how well the Delta variant spread (calculating R0) against the original SARS CoV2 under the same conditions. But we can’t do this.
We only know how the original SARS CoV2 behaved when everyone was susceptible, no infection control precautions were in place and we weren’t testing enough people to know who had Covid-19 and who didn’t. With the Delta variant we have most of the UK population who have either had Covid-19 or who have been immunised, we have had varying degrees of lockdown and infection control precautions and we have the capacity to test hundreds of thousands of people every day. It cannot “really” be said to be an equal comparison and therefore it is impossible to say if the Delta variant is more transmissible than the original SARS CoV2, or any other type of SARS CoV2 variant for that matter.
So, what do we know about the behaviour of the Delta variant compared to the original SARS CoV2? A recent paper from China has given some tantalising clues about how the virus is changing…
Shorter incubation period
The research paper from China looked at 167 patients with the Delta variant of SARS CoV2 and then performed daily quantitative PCR tests on all the quarantined contacts of those cases. Having been identified as a contact and been put in quarantine there was no further subsequent exposure to SARS CoV2. The researchers calculated the time from exposure to positive PCR as well as the amount of RNA in the positive tests. They then compared the results back to historical values from the original wave of infections in the same area of China.
The first important finding was that people infected with the Delta variant had a positive PCR by day 4 compared to day 6 with the original SARS CoV2; the average incubation period was 2 days less for the Delta variant.
This is really important as a shorted incubation period means that someone is infectious earlier than expected. Now, everyone who has been exposed to someone with Covid-19 is identified and told to isolate, but this takes time and time matters. Look at the time taken to identify contacts below:
- Pre-symptomatic infectious period = 2 days
- Symptomatic and tested
- Turnaround time for test = 1 day
- Time taken to identify contacts = 3 days (in my experience, this tends to be the minimum time)
So, we have a period of 6 days from someone being infectious to identifying their contacts who are at risk of becoming infectious and continuing the chain of transmission. If the incubation period is also 6 days, then test and trace should just about identify infectious contacts in time to limit further spread BUT if the incubation period for the Delta variant is now 4 days then contacts will have been infectious for 2 days before they have been traced! AND these 2 days are usually the period of time BEFORE the person develops symptoms during which they are most infectious!!!
So, it “looks” like the Delta variant might spread faster than the original SARS CoV2, but it could be that our current test and trace system is insufficient to stop it spreading.
Another important discovery by the researchers in China is that they found that people who developed Covid-19 with the Delta variant had 1000x more RNA in their samples than those with the original SARS CoV2. Now this doesn’t mean that they definitely had 1000x more infectious virus in the samples as the RNA could just be “fragments of viral genetics”, but it is suggestive of more virus.
So, if these people with the Delta variant of SARS CoV2 have more virus in their respiratory tracts when they cough, sneeze, or speak, they are exposing their “contacts” to more virus per given time frame. In order to test this study’s observation, the samples would need to be confirmed with viral culture methods in order to grow the virus. However that is too difficult at the moment with all of the biological safety legislation currently in place that prevents most laboratories working on live SARS CoV2. If this observation is correct, it might make the time of a “significant exposure” or “contact” LESS for the Delta variant than expected; meaning that 10 minutes might constitute a significant exposure to the Delta variant rather than the current figure of 15 minutes. This would mean some “significant contacts” of the Delta variant would not be identified by test and trace and therefore would be missed as their contact would be “thought to be” too short a time frame to trigger a ping.
Surely, we can now say the Delta variant is more transmissible?
So, the team from China have shown that the Delta variant may have a shorter incubation period and higher viral loads than the original SARS CoV2, surely this mean it is more transmissible? Well, the answer is still “we don’t know”. It might be, but we still don’t know the R0 value for the Delta variant, and… we never will. It could well be that despite having a shorter incubation period and higher viral load in the upper respiratory tract the Delta variant may have the same R0 as the original SARS CoV2 virus… it may be the same as the original SARS CoV2 and just as transmissible.
Saying the virus is “more transmissible” implies the virus is the problem when it may actually be that the infection control requirements need to be changed to suit the variant’s characteristics. Infection control is not a “one size fits all” response. Let’s look at a totally different scenario to help tease this out. The infectious period for chicken pox is 10-21 days after exposure, so contacts isolate during this time (what they only isolate on DAY 10?! …Yep, they really can continue about their normal business for the first 10 days) whereas the infectious period for Ebola is 3-21 days. If we “applied” the chicken pox “infection control rules” to a person who had been exposed to Ebola, then there would be a 7-day period where they could be infectious and spread the virus before we did anything to stop it… it would be carnage! When viruses change (or variants of viruses emerge) the infection control precautions need to change with them otherwise infections get out of hand.
What we are starting to find out though is that the current recommendations for managing contacts and test and trace may not be appropriate for all variants, but having different rules for each would be horrendously confusing.
It may be that the guidance has to move to the “lowest common denominator” and apply stricter criteria to all variants. This would mean the test and trace system would have to become faster and more efficient too… but I doubt that will happen as these large organisations are often just too cumbersome. Added to this, other “significant” large organisations (those who have the ear of those who run the economy) want the test and trace system to be “downgraded” or abandoned altogether.
In the meantime, let’s hope that the current fall in case numbers in the UK can be sustained and isn’t just due to the end of Euro 2020 or, more concerning, people deleting the app and not bothering to get tested anymore. It really would be nice to see hospital numbers start to drop again….
Yes… I am pedantic… the correct term to use instead of “transmissible” is “fitness”… I’ll tell you more in a future blog… grumble, grumble, mutter, mutter… I'm off to do a Spin class to improve my own fitness!