Woah woah woah, let's hang on for a second.
The Moderna vaccine candidate appears to trigger an antibody response in the elderly. That is promising, given that an antibody response to this vaccine protected macaques against SARS-CoV-2 in direct challenge trials, but it does not prove that it is effective.
The only way to prove efficacy is to start giving both the vaccine candidate and a placebo to people and then tracking how many people who got the vaccine get COVID-19 vs. how many people who got placebo got COVID-19. That is going to take some time.
One thing about immunity, there is re-infection of the virus in many cases, however does the disease present in the same person more than once? You can be reinfected with the same flu strain also but are you actually going to get seriously sick again or is it going to be mild or not even noticeable. Not all immunity is sterilizing like the Measles or Chicken Pox.
Regarding Coronavirus re-infection, it seems that someone can be reinfected but has a very mild illness or is asymptomatic. Sterilizing immunity might be short lived but remember if there is no natural immunity then a vaccine is doubtful. From the trials is seems that many of them produce both antibody and T-cell responses which is good news.
You get it. When people do get reinfected, they usually are found on an incidental screen (like the 33yo gentleman in Hong Kong who was screened on return from travel). They usually get minimal to no symptoms. There was, however, the once case of a young man (25) in Reno getting it twice with two different genetic variants. The second time, he got much more sick. He was never tested for antibodies after the first illness, but about two weeks into the second, he tested positive for both IgG and IgM. I think my best interpretation, given that his initial infection had been over 40 days prior, is that he didn't make antibodies the first time around. If he had, he would probably not have tested positive for IgM, which is usually only present for the first month or so after exposure to a new infectious agent.
It happens. Some people, particularly young people, can clear a viral infection entirely by using innate immune mechanisms that don't involve a memory response. This is common in norovirus, in which 30% of people don't get a memory response. Even for chickenpox, very rare cases of people getting the disease twice have been reported.
The other possibility is that this was a rare case of antibody-dependent enhancement. That would be more worrisome, but it still appears rare, and even if it was, ADE only occurs once per virus.
Herd immunity doesn't work with something that can reignite 3 months after being defeated.
Sure it can. Not all immunity need be sterilizing. When the most recent "common cold" coronavirus, HCoV-OC43 first appeared in 1890 it caused a global pandemic. They thought it was flu at the time but it appears it was something entirely different. HCoV-OC43 still circulates today. You've had it. I've had it. We've all had it. We got it when we were younger than ten, because kids are good at fighting respiratory coronaviruses. We got memory immunity, but not life-long sterilizing immunity. So we keep getting reinfected through our lives, but those reinfections are mild (common cold or no symptoms at all). Occasionally, however, those common cold coronaviruses can make people really sick.
If there were no vaccines, this virus would work its way through the population and settle into being an endemic common cold coronavirus.
Remember, there could be a worldwide pandemic of a virus that makes your eye twitch. But because it doesn't make people very sick, we'd never define it as a "pandemic." There is some context here. There are ongoing "pandemics" of common cold viruses, but because they rarely cause severe disease, we don't shut down society over them.