I guess I see things differently. I feel any good article will talk about the positive and negative. This vaccine is all we have. It saves lives.
Maybe it's all you have, as far as you see it, but that's definitely a personal circumstances kind of thing. Each to their own; it's not all I have and it saves no lives here - because I haven't lost a chicken to MD since I began breeding against susceptibility, and I never vaccinate for it. Whereas those who vaccinate tend to keep losing some. But some people really are stuck between a rock and a hard place, genetically painted into a corner so to speak, they can't stop. A non-leaky vaccine would be many prayers answered, I'm sure. What a terrible situation to be in, especially for rare breed enthusiasts and those who are facing losing their life's work as the vaccine loses relevance and the strains get stronger.
Back in the early 1900s they were already performing successful tests that proved it's easy and effective to breed for resistance to MD; harder to breed a bird that overproduces (meat or eggs) at the expense of its own health and still remains resistant, I am guessing... Whatever their reason, the commercial producers at the time chose not to go ahead with implementing the breeding programs on a wider scale despite the research outcomes. I've read that they chose the (seemingly) easy route, staking their hopes on the vaccine, instead. Sure sounds and looks like it.
It was a big step for science to be the first vaccine to stop most cancerous growth, which is so valuable to people. It continues to be worked on and improved.
Actually a lot of immunologists and vaccine makers ( have the links somewhere but probably actually quicker to google it up lol) are discussing (even publicly) not making any more vaccines because it's only making the disease stronger and stronger.
They have failed to 'improve' the vaccine, as far as I've read; only ever made new vaccines that address variations in strains, which are only getting more virulent; a series of escalating failures basically, each beginning with a bandaid cure that temporarily halts losses only to lead to ending up with a worse situation than they started with.
I guess it all depends how you view it; perspective is everything, but it's looking bad enough from enough angles for the very makers of the vaccines themselves to openly air the same opinions that are being shared in this thread.
If the usa was losing 2/3 of it's commercial chickens, and now the losses are less than 5% in commercial chickens with the vaccination, that's a miracle. It must be darn hard to plug the leak (continues to allow exposure to spread to others).
A lot of factors went into losses, it's not as simple as saying it was just the vaccine that stopped them; MD was as innocuous as ever outside of factory conditions, with extremely low mortality --- so what exactly were they doing to their factory reared chickens to lose such numbers?
Bad breeding was undoubtedly a massive part of it. Bad diet and unethical conditions probably also large parts of it. Some people were knowledgeable about poultry health back then but plenty had no clue and regulations were slack or practically nonexistent; case in point re: cluelessness, epidemics of coccidiosis wiping out large percentages of flocks, being a 'terrible scourge'. That, in my opinion, is primarily a disease of mismanagement, and also partly genetic in basis.
No doubt the vaccine has played a part in allowing stronger strains to develop. I erred in thinking the vaccine was created in the 70's. It was the 50's. However, the usa was losing massive amounts with the vaccine during the 70's. Turns out commercial producers looked to the Amish . The Amish practiced all in all out, not mixing ages, disinfecting between batches.
Which in my opinion is just another error, the all-in/all-out method... Can't breed for resistance that way, you're setting yourself up for failure that can only be delayed as long as you can balance in a precarious position. The more we protect, the weaker they get. Some protection is necessary but nowhere near as much as some flocks are subjected to.
If they did come up with a perfect vaccine/non-leaky vax that eradicated the disease, it would be worth testing, but not adopting large scale until we know what other immunological problems arise in the absence of the extremely low level 'common cold' type threat MD poses to the average chicken, only killing the very most susceptible and not even bothering the average individual.
Removing common, weak challenges to the immune system is now known to send autoimmune disease rates soaring. Some doctors and vets are now saying in the next thirty years we will be experiencing 'catastrophic' rates of autoimmune diseases of all sorts, given the escalating trends we're on right now, with nothing standing in its way, and some vaccines only making it worse. I believe weak diseases that only kill very minor percentages should never be vaccinated against given the fallout that later plays out over future generations due to loss of that challenge. We don't yet know enough about immunology to prevent this from happening; we're removing pebbles that are holding a flood at bay, slowly but surely.
So, is part of the problem leaky vaccine or all of the problem? Are there instances where vaccinated chickens do not spread the virus? Is this leaky theory really a new theory? Or just a new name for something that happens that's been understood for years already?
It was totally unacceptable for scientists to air such concerns about vaccines among most circles, they were shot down for it; history is full of examples made of those who tried. Awareness of the risks of vaccines has been as repressed as any unpopular opposing religious dogma ever was LOL. But the world is becoming more open-minded as the average layperson becomes more educated and scientifically-minded, and now it's becoming okay --- inevitable, even --- to address problems science has known about for decades. Progress, finally.
What research has already been done to reduce transmission?
Nothing much I've read of... But in saying that I do recall some articles I read, which I have bookmarked somewhere, discussing ways to try to prevent proliferation in the follicles, but I don't think they were referring to using a vaccine for it, more theoretical potential treatments, some of them quite toxic. The usual all in/all out, mainly, is of course still what people are relying on.
You should not be so worried. Vigilant, yes, and keep records, and watch to ensure you are breeding stronger rather than weaker lines --- but doesn't that apply to any breeder worth their salt? In short you'll just be following mostly the same protocol decent breeders of the past followed, trying to breed for hardiness, fertility, temperament, type, productivity, etc, (as applies to your flock/breed/needs and aims) and disease concerns won't plague you like they do those only breeding for type or productivity at the expense of the other vital traits. 
But the garlic and turmeric is definitely a start. 
