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There isn't a lot of research out yet about this, because it's all so new, but early reports show that COVID antibodies from previous infection aren't as effective as vaccines in terms of protecting against the Delta variant. A new study in Nature shows that in 56 people who had COVID, their "natural" antibodies didn't protect very well against Delta, but after even just a single dose of Pfizer or Moderna, they neutralized all 4 strains of Delta variant.
There isn't a lot of great data on the full length of natural immunity, or the immunity someone gets after they get sick with COVID. There is strong data showing that it lasts at least 90 days, and more recent studies showing it may last even longer, up to 8 or 9 months or more according to a recent NIH study. There's good evidence that those who were previously infected with COVID and vaccinated have very high protection levels after even just a single dose of mRNA vaccine. The CDC is still recommending they get a second dose just to be safe and to keep the recommendations consistent for everyone, but it does seem those with previous COVID infection and two doses of vaccine are basically the best protected people out there. It's important to note here that there was a recent study showing that for people who have never been infected with COVID before, it's crucial to get BOTH doses of vaccine for Pfizer and Moderna in terms of protecting against the Delta variant - one dose doesn't do much, but two works well against Delta and other variants.
No - while breakthrough cases are happening, the vast majority are mild and don't require hospitalizations. The CDC has heard of about 5,500 total cases where someone vaccinated has gotten sick enough to require hospitalization, which is a tiny percentage of total cases - compare that to over 605,000 deaths and so many hospitalizations that the CDC can't even track that number. Most of those severe breakthrough cases are in people over age 65, and many have other comorbidities which may have added to the severity of their illness. The vast majority of those in the hospital and dying of COVID now are unvaccinated.
Not really. Breakthrough cases are rare, but they do happen. There’s some evidence that the J&J vaccine is slightly less effective against mild to moderate illness, but all three vaccines approved in the US are extremely effective against severe disease. One thing to keep in mind is that there are over 337 million people vaccinated, so while breakthrough cases are rare, these numbers are so huge that even a tiny percentage of breakthrough cases is still a lot of people. If 95% of people vaccinated are totally prevented from getting COVID, that still leaves 5% - or 16.85 million people - who might get sick, though their illness will be much milder than if they weren’t vaccinated at all. And remember, the vaccines haven’t been administered evenly - nearly half of all vaccines given are Pfizer brand, because they were first and able to ramp up production sooner. So if you hear of a few Pfizer breakthrough cases, don’t assume that means Pfizer isn’t working, because it absolutely is.