Chills, headaches and muscle pain were reported more frequently when different vaccine doses were combined."
Lots of people have those with any of the vaccines.
Sounds like it's an improvement actually - no mention of tiredness or feeling some what off such as mildly dehydrated.
Then how did they combine doses?
Also of course forgetting why the vaccines were available a lot sooner than usual. They were produced much more quickly than they usually are. That aspect often takes a long time. Testing in real terms is no different to what it would usually be done with others. They need to come up with it in the first place. Then establish that it's unlikely to harm people. Then seek volunteers who are prepared to take it. If that proves ok offer to more people and watch what happens. In covids case there is plenty of people to test it on, it's active so results come in quickly. Still ok - offer it to the wider population. All vaccines can have unfortunate effects on very low numbers of people - just like most medications of any type.
Why do people get headaches etc. In some respects due to their immune system being active. Some of the curious effects are purely down to that.
AZ and clots. The Norwegians tried to look at how many occur naturally. It varies so they used and average. Result - there may be an association with the vaccine. It's a tricky area as some people may be more prone to it - there are theories about that. It's a bit like the main test results. The people and their chance of catching covid are random events that can't be controlled. This is why they would even be interested in vaccines that showed just 50% effectiveness. Now millions have had them the results in terms of effectiveness will be far more reliable.
Flu vaccines - they eventually found a way of weakening a variety of flu virus. Different ones need to be used each year.
Smallpox - anecdotally some found that that puss from some one who had it could give people immunity. Like covid it didn't kill everybody. Eventually they noticed that milkmaids didn't catch it - read people who spend time with cows so the vaccine is based on cowpox.
AZ - based on a virus that gives chimps a cold and doesn't do much at all to humans. Genetically modified to cause our immune system to react in the right way. Impossible when other vaccines were developed.
Pf - totally artificial. The same technique has been in use for some time now to try and defeat cancer and similar. There was a breakthrough not all that long ago in the area of delivering it more effectively.
All of them are intended to get our immune system to respond in the right way. Part of the reason for people feeling ill when they have viral infections is the fact that the immune system is active. Same with some of the side effects from vaccines. Some peoples immune system works more effectively than others - especially the
very young. Some people's remain more like that for longer than others. It's why kids tend to recover more quickly than adults.
Last but not least. If you haven't caught it already there is a sporting chance that you will in the future. Risks from the vaccine runs at 10's in millions. Covid risk is way way higher right down the age range. It's not just death. Hence the shear number of people in need of treatment for after effects.