I have been using this spray, or different versions thereof, for decades to avoid getting colds. I still get colds, but not many. I also know I avoid colds when I should have gotten them. 

It is basically saline with red seaweed extract (carrageenan). I ignore the maximum dose guidelines, and may use many times in a compromised situation e.g. on a plane, in a work environment when others are sick etc..

You have to use it within 1-2 hours of being exposed to a situation where you could get a cold or similar, or when you start to get the scratchy feeling in your throat that tends to precede a cold. If I am out with a group of people and one is sick, or I am out in public I use the spray when I get home.

I was so convinced by this as a potential prophylactic for covid, I shared it with the Chief Medical Officer of the UK in early 2020. I never got a response. I am sure it was just classified as another crackpot theory. 

But clearly, someone worked it out. This is the science that proves there is something in it. It is just one of numerous studies that demonstrate this. Quote: "a nasal spray with I-C [carrageenan] showed significant efficacy in preventing COVID-19 in health care workers managing patients with COVID-19 disease."

Another thing that came out during Covid was that certain mouthwashes may also reduce the risk / effects of Covid. One of these is Cepacol. So if I start to feel iffy, I also gargle on this. Cepacol is not designed for gargling, so it foams when gargled, and it feels like it goes up the sinus cavity a little, reaching parts the nasal spray does not. Anyway, this seems to help. And, good oral hygiene is increasingly being seen as reducing the risks of dementia. So if you do get covid, you won't forget about it in your old age.

This is all amateur science, and you should follow anything above at your own risk, it absolutely works for me, to the extent I am prepared to sound a little crackpotish and share it. 

Whilst I got very mild covid, I did not get cold or flu during the winter after the pandemic, whilst everyone around me did. I am convinced it is because I followed these routines.