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The latest news from the US is that the CDC & FDA recommend a booster shot after only two months following the "single dose" J&J vaccine.

Given the Fauci comment that J&J "should have been a two-dose vaccine to begin with", I looked through the initial preclinical data of J&J and sure enough there's this:

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Spreading the same total vaccine amount two doses improved titers in macaques. So, do we know anything why J&J went for the "single dose" thing? I mean is there any later clinical data in which there wasn't much difference between one two doses of J&J?

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    Possibly more of a strategic business decision? They knew what Moderna and Pfizer were doing, so being the only one with a single-dose option would be attractive to a lot of customers.
    – Carey Gregory
    Oct 23 '21 at 3:50
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According to a March 2021 report quoting an infectious disease expert from the University of California San Francisco, Dr. Monica Ghandi, the J&J (Janssen) vaccine was proposed as a one-dose vaccine because:

“After one dose, across all populations, even in older people, the antibody response and T-cell response were excellent and increased over time.”

https://www.ucsf.edu/news/2021/03/420071/how-effective-johnson-johnson-covid-19-vaccine-heres-what-you-should-know

In the same article, Dr. Ghandi also pointed out that an advantage of a one-dose vaccine is that:

“on a population level, the availability of a one-dose vaccine can really speed up vaccinations and help bring total cases down.”

In a January 2021 commentary in Nature, an infectious disease expert at the Medical University of South Carolina, Dr. Krutika Kuppalli, described some logistical advantages of a single dose vaccine in certain populations as follows:

“And bringing vaccine recipients back for their second dose can be a logistical challenge, particularly with people who are homeless, who use drugs or who live in rural areas”

https://www.nature.com/articles/d41586-021-00119-7

A February 2021 CNN report quotes a spokesperson for J&J quoting the World Health Organization as follows:

"A one-dose vaccine is considered by the World Health Organization to be the best option in pandemic settings, enhancing access, distribution and compliance."

https://www.cnn.com/2021/02/23/health/johnson--johnson-covid-19-vaccine-fda-committee/index.html

Thus, J&J might have had a business motivation for making a one-dose vaccine looking at the anticipated world market, although this is speculative.

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