Immunisation works at two levels and the population effect depends on how many people participate.
Individual protection
Reduced risk of infection and of severe disease.
Which varies by vaccine and by disease.
Population protection
Reduced transmission protecting those who cannot be vaccinated.
Which requires a coverage threshold.
Waning and boosters
Protection declining for some vaccines.
Safety monitoring
Surveillance continuing after licensing.
Which has identified rare effects and led to changes.
How the population effect works
When enough people are immune, transmission chains break, and those who cannot be vaccinated because they are too young, immunocompromised or allergic are protected indirectly.
Which is why coverage thresholds matter and why falling coverage produces outbreaks before it affects the majority.
Those thresholds differ by disease according to how transmissible it is, and highly transmissible diseases require very high coverage.
Individual against population benefit
Both real and operating differently.
Which is why programmes target coverage rather than individual uptake alone.
Safety surveillance
Monitoring continuing indefinitely after licensing.
Which has detected rare effects.
Schedules
Timing set by immunology and epidemiology.
A general note
Individual decisions should be discussed with a healthcare professional.
Why knowing how the system works matters
Most frustration with healthcare comes from processes that are entirely rational from the inside and completely opaque from the outside. Why reception asks what is wrong. Why one person waits longer than another. Why discharge takes a day after the doctor said you could go. Why a result outside the normal range is not necessarily anything.
None of those is arbitrary and none is explained at the point where the explanation would help. The people running these systems are solving clinical and logistical problems, and the patient experience of those solutions is a by-product nobody designed.
What patients can actually do
Give complete information when asked. Report deterioration rather than waiting. Ask what a recommendation is based on and what the alternatives are. Request a second opinion when the stakes justify it. Read your own records at least once. Use the service that matches the severity.
None of that requires medical knowledge, all of it improves outcomes, and most of it is never suggested to anyone.
The things nobody mentions
Second opinions are routine rather than confrontational. Records are accessible by right. Complaints procedures are free and independent at the escalation stage. Interpreters are provided. Advocacy services exist. Carers have entitlements they mostly do not claim. Pharmacists can treat a great deal without an appointment.
Every one of those is published somewhere by the relevant health system, and almost nobody encounters the information until they need it and are least able to go looking.
A general note
Nothing here is medical advice, and health systems differ enormously between countries in structure, entitlement and process. Anything concerning your own care belongs with a qualified professional.
The pattern across all of this
Health systems are built around clinical priority and constrained capacity, and almost every process that frustrates patients follows from one of those two things.
Triage exists because some people need to be seen sooner than others. Waiting lists are ordered by urgency because treating chronologically would harm people. Discharge involves several organisations because care at home requires more than a medical decision. Tests are ordered selectively because testing indiscriminately produces false alarms and unnecessary investigation.
Understanding that does not shorten a wait. It does remove the impression that the system is indifferent, which is the interpretation most people reach in the absence of any explanation.
On dealing with staff
People working in health services are generally operating under pressure, within constraints they did not set, and dealing with a great deal of frustration properly directed at the system rather than at them.
That is not a reason to accept poor care, and complaints procedures exist precisely for when something has gone wrong. It is a reason to separate the two, which produces better outcomes in both directions.
Where the reliable information is
Health services publish patient information covering exactly this ground, free and written for the public. Regulators publish standards and inspection reports. Guideline bodies publish public versions of their recommendations. Advocacy organisations help people navigate all of it.
All of it is available before anyone needs it, and almost nobody reads it until they do.
One last point
Almost everything described here is checkable, published and free, and the moment when someone needs it is generally the moment when they have the least capacity to research it.
Spending half an hour understanding how the system where you live actually works, before you need it, is among the better uses of that time available. It is also the sort of thing nobody does, which is why the same misunderstandings recur.
Where to look
National health services publish patient information covering access, rights and processes, free and written for the public. Regulators publish inspection reports and standards.
Both are more useful than general search results and neither has anything to sell.