A shameless plug here for Infowars Stores products.
I had stocked up on many different supplements from Infowars back a couple years when I still had some cash.
Selenium was one of them ... their formula at 1 capsule a day is 286% of daily recommended. So one cap a day is gonna give you a good selenium boost. ( I took 3 immediately after reading the selenium article. Hadn't been taking it regularly for quite a while.)
Other supps which will be good for immune boost is the DNA force and or Cell Force ... both are same save for a few more extra ingredients in the newer Cell Force. (Kinda expensive but has been reduced from much higher price in the past). Vitamin D3 is also important and they have a good one also their 95% curcumin extract, Bod-Ease.
I have been using these products and many more (oh almost forgot Survival Shield X2 & X3 Iodine) for a while now and they are as advertised ... very good. And if you catch a good sale which happens all the time, are reasonably priced. Even for me here in Canada with the dollar exchange and shipping cost.