America is seeing a huge improvement in vaccine shipments

This week, with 2.8 million doses of the newly-authorized Johnson & Johnson COVID-19 vaccine in tow, 17.6 million coronavirus vaccine doses in total will be shipped out across the United States, a marked improvement from the 10 million doses the country was averaging just a month ago.

States are also getting those shots into people's arms more quickly now. On both Saturday and Sunday, more than 2.3 million received a vaccine dose, suggesting that at least 3 million people could be getting a shot daily by the end of March, The New York Times reports.

Now, there's cautious optimism among experts that herd immunity could be on the horizon, and the single-dose Johnson & Johnson shot appears to be a major reason why. Per Vox, if one-third of vaccines are one-shot and the U.S. is administering 3 million doses per day, the U.S. could reach 80 percent immunity by mid-summer.

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The Mayo Clinic's Vincent Rajkumar estimates that, at this point, around 140 million people will need to be vaccinated to reach herd immunity, a goal he predicts could be achievable within three to four months. Rajkumar said his estimate is likely conservative since the actual number of people who have already been infected is probably much higher than what's been recorded.

There are several caveats, including lingering vaccine hesitancy and uncertainty over variants' ability to resist immunity, but the U.S.'s much-maligned vaccine rollout looks to be on the upswing.