Mumps, measles, rubella...
The hard vaccines are the ones that haven't been done yet. SARS-CoV-2 is brand new so it may turn out to be easy.
Those have a respiratory component and are airborne diseases, but they do their work in other parts of the body. This page has a list of lung infections:
Respiratory Illnesses: 13 Types of Lung Infections
We do have a vaccine for whooping cough, but that's a bacterial infection. Work has been done on a MERS and SARS vaccine. The SARS vaccine can't be tested because no new human cases have been seen in over a decade.
Many people think coming up with a vaccine is like building a new highway, it might be expensive and take some time, but it is a well know procedure. Just about every new vaccine requires advancing medical science as the first step, then try a large number of approaches and hope one works. Then when a promising candidate comes to the fore, they have to do lots of testing first to see if it works on animals without side effects, then run several trials on humans in ever increasing population sizes. Not only are they looking to find a vaccine that prevents the disease, they are also concerned about side effects. To find a vaccine that protects you from the virus, but causes 10% of people who get it to have a fatal heart attack is too dangerous to use.
Studying a vaccine is also tough to do and takes a lot of time because it is unethical to expose someone who has been vaccinated to the virus (in the early days of vaccine research they did do this), so you have to rely on people encountering it in the wild. If someone gets the vaccine and doesn't get sick, you don't usually know if it's because they are now immune or if they never got exposed with enough of the pathogen to get sick if they haven't been vaccinated. We know that some people have been definitely exposed to COVID-19 through contact tracing, but never got infected. Through luck of the draw they didn't get the virus into the critical places to get the infection.
The early human trials of the COVID-19 vaccine are more looking for side effects than whether it works because there are too few people in the study to ensure many are going to be legitimately exposed.
Then once all the testing is done, scaling up production will take time. Coming up with enough doses to cover just the United States is a monumental task bigger than any previous vaccine roll out. It could take more than a year to scale up to enough doses to cover everyone.
The fastest a vaccine has ever been developed to date is 5 years start to full production. There is a lot of talk of shattering that record with a COVID vaccine, but that might just mean cutting the record in half.
We have some candidates now, which is very quick, but we have no idea how long immunity lasts if it does work. If the immunity is very short lived, like 2 months, it will be vastly more difficult to produce enough vaccines to cover everyone because it will require 6 shots a year per person and to cover a country like the United States requires 1.8 billion doses a year pretty much forever.
And what if the only vaccine that works costs a lot to make?
This could become a haves get the vaccine and the have nots are left to suffer and die.
Like most things these days, the US doesn't make many vaccines. 60% of world production is in India. To make a home grown vaccine is going to require rebuilding the expertise in this country to make vaccines again.
Most industrial processes include institutional knowledge that isn't written down anywhere. If you get the right people who know that institutional knowledge from somewhere else, you can flatten the learning curve, but the expert needs to teach a bunch of newbies how to do it and that takes time. Tesla suffered from this and didn't really get down the art of dinosaur car building tech (bend metal and welding it together) to a level on par with the rest of the industry until about a year after the Model 3 was in production. In many other areas the rest of the car industry is trying to catch up with Tesla on industrial knowledge, but Tesla had to learn some things the hard way.
Biological processes are probably more difficult to control effectively than a traditional production line. A minor contaminant in a batch of product being grown can ruin the whole batch and can literally kill people if not caught. Biological processes also involve a lot of waiting for things to happen at their own pace. Each year's flu vaccine has to be started a year before the first person gets vaccinated.
We may have a COVID vaccine and we may have it in record time. We may get lucky and it's both cheap and easy to produce. But we may not and it may be very difficult to produce and/or it may take years to come to market. There is also the possibility that we may never have a viable candidate for a vaccine.
Over the last 100 years we have managed to create vaccines for the worst of the diseases that have plagued humanity through most of our history. Most of the people here have lived their entire lives in the tiny blip of human history when the developed world really didn't have to worry that much about infectious diseases. People would get hair on fire when there was a measles outbreak or a few people caught the bubonic plague from some animal, but the real risk to most of us was small.
Last year there was a "major" measles outbreak in my county. I think something like 65 people got sick and there were a few cases spread around the region. Compare that to 1940 when there were 600,000 cases in the US. Even with anti-vaxers leaving their kids at risk, the magnitude of outbreaks is several magnitudes smaller than they used to be.
We have been lulled into thinking that medical science will always solve the problem and it will become a minor threat or none at all. We've done a pretty good job with the old diseases and some of the new ones like HIV/AIDS are in decline in many parts of the world (by driving the r number way down), but there is no guarantee that this one will be solvable with just a shot. It
might be, but there is no guarantee.
We live in interesting times.
