@gzelvis
https://en.wikipedia.org/wiki/Vitamin_C#Treatment_of_the_common_cold
Vitamin C's effect on the common cold has been extensively researched. It has not been shown effective in prevention or treatment of the common cold, except in limited circumstances (specifically, individuals exercising vigorously in cold environments).[64][65]

https://en.wikipedia.org/wiki/Vitamin_C_and_the_common_cold
In the 1970s, Linus Pauling argued that vitamin C could significantly decrease the incidence of the common cold, which spurred a widespread belief that consuming more vitamin C will reduce the risk of catching a cold and reduce its severity......A subsequent meta-analysis found only very minor reduction in duration, and the authors concluded that vitamin C had no value in treating the common cold.[5]

这想 wikipedia 说的够明白了吧。

事实上，在医学上想证明一件事，通常要做两件事。
1. 机理。任何药物要首先从机理上说的通。而维 C 能治感冒的机理是什么呢？
2. 医学实验。从目前为止的实验上，研究者得到了 vitamin C had no value in treating the common cold.

另外，对于一个不了解临床实验的个人来说，可能会把某次实验，某篇论文，或者类似 individuals exercising vigorously in cold environments 有 benefit 当回事，但医学界并不会因为某篇论文就改变意见。做一个临床实验，会测几百种生理指标，只要测的指标够多，总有指标看上去有疗效的，这是概率问题（比如 p value=0.05 的话，只要条件组合够多，总有一两项指标符合统计学，但这种“发现”下次的实验里根本不能重现）。