To keep healthy, human bodies require nutrition, activity, and rest. Nutritional substances required to stave off respiratory infection would include things like vitamin C, vitamin D, and zinc, among others. Here are a couple of suggestions for natural resistance:
The Wellness Company
Signature Series Natural Immunity Formula
+
Liposomal Vitamin C
+
Optimized Vitamin D3 and K2
And for those on a tight budget …
Now Foods
Respir-All
+
Vitamin C-1000 Complex, Buffered Tablets
+
Mega D-3 & MK-7 Capsules
In either of these triple-combos, you get good amounts of key immunity nutrients. If combined with a diet that avoids excessive insults to the immune system (e.g., excessive processed foods and beverages), they might be able to keep you from getting sick, even in Winter-time
Reference
Immune support from The Wellness Company. https://www.twc.health/collections/covid19
Immune support from Now Foods. https://www.nowfoods.com/products/supplements/health-need/immune-support
Sabine Hazan (https://twitter.com/SabinehazanMD/status/1639852682008727553/video/1) talks about how the vaccine destroyed bifidobacterium in the gut.
Every day I eat some yogurt with bifidobacterium in it now.
Can you call it "natural" if you are taking supplements?
My suggestion would be a diverse diet of minimally processed foods and sunshine. :
Just to take Vitamin D as an example: although Vit D levels correlate with skeletal, cardiovascular and neuropsychiatric health, raising the blood levels to normal by taking oral supplements is less effective than raising blood levels through sun exposure in improving cardiovascular and neurological health, though approximately equivalent for bone health. Suggests that while Vit D levels are a good way of measuring sun exposure, the vitamin (which should rather be called a hormone) is only a part of the story in terms of the health benefit of sun exposure.