[Lewis Thomson (OP)]

Jackie has stumbled across this unusual circumstance

"My brother and his wife are staying with my husband and I over Christmas, my husband and sister in law have had a heavy cold (negative daily LFT) my brother and I contracted covid, however we have all been together. Is it likely that they have not developed covid as a result of having a cold which prevented the virus being ingested as easily?"

Is this the case? Leave your findings in the comments below...

[evan_au]

The classic "cold/flu symptoms" are actually a side-effect of Interferon, a signaling molecule produced by your body when it detects a viral attack.
- Interferon activates the immune system
- Interferon also slows down some normal cellular functions, so the virus can't hijack them

A unique "stealthy" feature of COVID is that it suppresses production of interferon by infected cells, so that it can spread "under the radar", infecting not only you, but those around you before symptoms appear.
- It makes sense that if your Interferon levels are already high due to the common cold, an incoming COVID virion would have a harder time gaining a foothold in your body

 See: https://en.wikipedia.org/wiki/Interferon

[evan_au]

a heavy cold

What we call "a cold" is actually the total impact of 150+ respiratory viruses that spread regularly across the population, without causing much disease or death.
- 4 of these "common cold" viruses are actually corona viruses, distant relatives of the SARS-COV2 virus that causes COVID-19
- The endemic corona viruses tend to come back around every 2 years, as immunity wanes.

Recent research in the UK looked at people in the same household - one caught COVID-19, the other didn't
- They looked at blood samples from these pairs, and discovered that people who had no infection with COVID-19 had immune T-Cells that reacted with the "common cold" corona viruses as well as COVID-19

So someone who recently (in the last year) had a cold which was one of these common coronaviruses would be more protected against catching COVID-19 than someone who had not been exposed for (say) 5 years.

[set fair]

Some viruses have the ability to inhibit other viruses but it is the property of the individual virus rather than the severity of illness that counts. Different states in the US are often unsyncronised in covid waves, but there have been occasions when every state saw a downward blip within a couple of days of each other - another respiratory virus spreading is a possible explanation.

Some infections can detect that the immune system is at defcom1 fighting another infection and then hide away until the immune system stands down.