The Arabian horse chats are going crazy with pros and cons of vaccinating for the West Nile Virus. One tale tells of a breeder (location unknown) who lost 1/3 of her foal crop. Since she'd vaccinated the mares, that was a suspect in the cause. I am unclear if the mares were past the time when the vaccine should have been effective...
But, here's my two cents' worth.
I've been breeding and foaling out horses since 1962.
When a foal that is born healthy and dies the next day, I suggest checking the amount of iodine in the soil/feed stuffs in the area...
One excessively wet year, (when I had given a then experimental vaccine) I started losing foals that were born just fine. The early foals, born while the mares were still mostly on winter hay, were fine. Then we put them out top pasture, which was thick and lush. They were out a bit longer each day, and finally out full time.
The first foal born about a week after the mares were all out in the pasture died the next day without a mark on him. His dam (still alive at 25), was the boss mare, so she got the bestest the firstest... and nobody crowded her. The death didn't make any sense.
The next foal born came so his second day was on a weekend when I could attend to him totally. The vet came out FOUR TIMES, but he died anyway. I still had two mares yet to foal.
"Take it to Ames (Iowa State University, the Veterinary Training School for the area). We shouldn't have lost that one. I'll call up. Go right to the diagnostic lab."
I did. Only students were on duty when I got there, so it was highly unsatisfactory to just drop off the body and go. (The foal had been promised before birth to a youngster, which made it doubly painful.)
A week went by. I was getting VERY nervous. The vet called -- iodine deficiency -- the thyroid was the culprit. I needed to start giving the next foals a teaspoon a day of (T...I forget the name- a supplement) and MIX IODINE into my salt/feed, etc. AT ONCE, for all horses on the farm.
I went into three different vet's offices in the area over the course of that summer, seeing huge stacks of 50 gallon DRUMS of iodine in them all. (I'd been sold a 5 lb. zip lock baggie of it...and I had over 20 horses at that time.)
"Who's buying in that kind of bulk?"
"Cattlemen." It turned out that cows were also susceptible to iodine deficiencies, but in a totally different way. It caused problems THEY would be in danger from.
I've always figured that MY $250 research bill saved countless cows, and my next two foals, both lovely Pinto fillies, in our area. (One filly I still own. One of their dams is now dead -- at age 21, so no lasting problems seemed to develop in the stock I kept, anyway.)
I'm sure there are probably many other reasons that youngsters born healthy die mysteriously, but that one sticks in my mind.
By all means, report the deaths to the vet who sold the vaccine to you, but don't be too quick to throw stones. (And, NO, I do NOT give the other vaccine, but not because of that year's losses...)