For complete herd immunity we'd need something like 100% that about 80% percent take, which is hard. So, our current goal is to get rid of the bad effects of Covid by limiting spread and limiting severe symptoms.

> But i still do not know what is the definition of "effective vaccine".

I'm not aware of a rigorous definition, but effective vaccine is something that helps us significantly to accomplish the goal. All the vaccines provide very good (90%+) protection from severe symptoms and it seems to be good even with the current variants. The protection is not as good against any kind of infection, but it is still very significant (like at least 66% even with some variants).

> If you have covid and take the vaccine, will the covid go away?

No, you have to take the vaccine before Covid. Usually it takes some time before the vaccine is efficient. Like you have to patch your server before it becomes part of a botnet.

> Are you still contagious after having the vaccine

You may be, but with significantly lower probability.

> can you still get the covid but without the bad effects?

This seems likely outcome of being vaccinated - your body will somehow already know what to do and will with the virus faster.

> What kind of immunity is given by the vaccine and how long does it last?

We'll see how long it lasts. It does not seem to be diminishing, so it should be at least a year or so. From the experience from other vaccines, it will probably last longer, but there is the risk of new strains that evade it.

The rigorous definition, for efficacy in any particular outcome, is one minus the effect estimate from an RCT, usually a relative risk or hazard ratio, depending on how they approached it statistically.

Well, based on my understanding, we don't know for sure.

What we do know, is that they give you a much higher chance of avoiding serious symptoms, including that most serious of symptoms, death. You won't be "virus-proof" completely, but someone vaccinated fully (so 2 doses for vaccines which require it), after a certain immunization period, which varies between 2-4 weeks after being fully vaccinated, will be super resilient to being in contact with Covid patients. Don't go get coughed on by 10 Covid patients, though.

Regarding contagion, apparently after vaccination, you can still get Covid with most vaccines, you just become "asymptomatic", i.e. it doesn't really affect you. However it seems that you're also much less contagious, which is still great, since it reduces the infection rate, it becomes much harder to spread it.

If you get Covid while immunity from vaccination hasn't kicked in fully... you can still be screwed. People have died 1 week after being vaccinated. So you probably want to stay put for a while until your immunity ramps up.

Regarding immunity, as I was saying, it's primarily lack of serious symptoms. I think they also make it harder for you to catch it, but I don't think we have super solid proof of this yet.

And for immunity, it's hard to say. We're basically just counting up days from initial vaccinations to see how long it lasts. For the other vaccines we know the time intervals because... time has passed. So far, for the current vaccines it seems that immunity is at least 6+ months. From what I've read we should be reasonably optimistic that we'll be immune for 1+ years. I don't think anyone expects lifetime immunity, yet.

TL;DR: Get vaccinated, it will keep you much safer, it will probably also help others around you, by making you spread it less if you do get it. You'll still get Covid but it will be much less of a deal. Side effects are temporary and not massive in the vast majority of cases.

I'll take a swing at this, being an infectious disease epidemiologist.

When people talk about the "effectiveness of the vaccine", what's actually happening is that they're talking about a smear of different things, which is why you're (justifiably) confused.

There's a few different aspects to think about:

1) When people say things like "The Moderna vaccine is 95% effective", the statistic they're citing is that in a clinical trial, the participants in the treatment arm of the trial had 5% of the outcomes the study was tracking as compared to the treatment arm. In the case of the early vaccine trials, this is stuff like...well...death or severe disease requiring ICU treatment.

It importantly does not mean you have a 5% chance if you get COVID. You have 5% of whatever the chance of someone who didn't have the vaccine has.

There's a reason that endpoint was chosen. A few actually. First, it's the most important one from a public health standpoint, in terms of immediately addressing the pandemic. It's also straightforward and unambiguous to study, which paves the way for faster licensing and thus getting it into the arms of the public. Studying things like if a vaccine is enough to induce herd immunity is much harder as a statistical problem.

Note that I haven't said anything about stopping the epidemic here. That's because this endpoint doesn't measure that. It's possible, for example, that the vaccine can protect you by reducing the severity of an infection, but that that would just push it to an asymptomatic infection that can be passed to others. That's why there's still public health messaging about wearing a mask even if you're vaccinated, etc.

2) There are follow up studies, which are still being done (and for which we are starting to get results) about whether or not the vaccine also reduces whether or not you can give the virus to others. This is what most people think of when we think about vaccines.

3) We don't know how long it will last, but it's likely fairly long lasting, and this is something that we can (and are) monitoring to make sure people don't need booster shots, etc.

4) Herd immunity depends on point #2 working well, and being at a fairly high level. One thing to keep in mind that I say a lot is that "herd immunity" is about a specific place at a specific point in time. It's possible for example that a city or town can have high enough vaccine levels to have achieved herd immunity, but the country (or the globe...) not being vaccinated enough for SARS-CoV-2 to have "gone away".

Public health thinks about this using a couple terms:

Control - "We've got a handle on this, but we've still got it" Elimination - "This is gone in our area, but could come back" Eradication - "This is gone forever"

TB is an example of a disease the U.S. has under Control. This is what we're aiming for with COVID-19 right now. Measles, Mumps and Rubella are, in most of the U.S., considered Eliminated. Smallpox is Eradicated.

Thank you for your clear comment. I hope your tenure review has a positive outcome.

Thank you and all others who chimed in for this most valuable insight and contribution.

Vaccines are ways to mimic infection (hopefully) without the side effects of real infection, like getting sick.

The immune system is complex and layered. The body's primary response to an infection is to make the body inhospitable. We know this as a fever. Some infections, like the common cold, are defeated here.

There are more tools available for more resilient infections, one being antibody production. Antibodies are proteins that bind to pathogens, to act as a marker. Once marked, other mechanisms (white blood cells) can clean up.

It takes time for the immune system to design an antibody that can identify the pathogen. The produced antibodies generally hang around for a few months, or longer. This is driven by chemical factors. Fortunately, Memory T-Cells will last a lot longer, and these contain the information needed to identify the pathogen and produce new antibodies. If the same pathogen appears again, antibodies can be produced quickly.

An antigen is anything that triggers an immune response. This includes pollen, if you have hay fever. In the context of COVID, the antigens are usually spike proteins, which stick out from the surface of the virus.

It's also worth noting that this process isn't perfect. Exposure does not mean immunity. Immunity is not binary, either. T-Cells may prevent symptoms entirely, or their may be a lag.

So, now I think I can answer your questions.

The vaccine provides the antigens using a form of COVID that won't, or shouldn't, cause infection. This will still cause an immune response, which may be strong enough to cause side effects. The body will see the antigens and generate antibodies. The antigen/antibody combination is stored in memory T-cells. This process takes several weeks, and is strengthened by a second exposure.

If you already have COVID, unfortunately the vaccine won't help. Vaccines are primarily preventative. There are a few diseases with post-exposure vaccination (anthrax), but there are usually many contingencies involved. If you have had COVID, you should still get the vaccine, because you are not guaranteed immunity.

You may still be contagious or symptomatic after having the vaccine, but this should (at least) be reduced. There is still a lag between becoming infected and immune response, even if you have immunity. This will be much shorter in the immune group, however.

It's hard to say how long immunity will last for. On an individual level, you can find pessimistic cases indicating very short immunity. Remember everyone's response is unique, and unlikely cases are usually most reported. The wildcard is a mutated virus evading immunity. To eliminate COVID would be to vaccinate the world, which is highly unlikely given substantial portions of the world haven't yet admitted they have it.

Assuming your chance of getting COVID is "100", two weeks after vaccination your chance of getting it will have dropped to "3" and your chance of dying from it will be "0". Changes to your ability to have a symptomless infection and transmit it to others aren't entirely clear yet, but seem to tend towards the "3" as well.

This is for the mRNA vaccines (Pfizer & Moderna). The others are potentially slightly less effective in preventing infection, but seem to be just about as good in preventing serious courses and death.