In the J&J trial, the placebo group had 16 hospitalizations and seven deaths from COVID-19, whereas the vaccine group had none, which means the vaccine provided 100 percent efficacy against hospitalizations and deaths.

For severe disease, which includes people who were sick enough with COVID-19 to require medical intervention but recovered without hospitalization, the efficacy was about 85 percent across the board in Brazil, South Africa and the U.S.

Including mild and moderate disease, the overall efficacy was 66 percent, but varied across the regions: 72 percent in the U.S., 64 percent in South Africa, and 61 percent in Brazil. “Mild and moderate outcomes” could include a range of illness, said Gandhi, and we won’t know the details until the full trial results are published, but we do know that everyone recovered without medical intervention.

“Admittedly, against mild to moderate disease, it didn’t work as well, and I understand people’s concerns,” said Gandhi.

Cars lined up in parking lot with worker in neon yellow vest
Cars lined up at the the City College drive-through vaccination site in San Francisco. Photo by Maurice Ramirez

Yet she said that on a population level, the availability of a one-dose vaccine can really speed up vaccinations and help bring total cases down. She pointed to the United Kingdom, which has rolled-out a similar one-dose, adenovirus vaccine, made by AstraZeneca, that protects against severe disease, hospitalizations and deaths very well but is less effective against mild and moderate disease.

“Their hospitalizations and cases are plummeting faster just because they have more vaccine supply. Their outcomes have been phenomenal,” said Gandhi. “So when I look at that, I think, ‘Oh, I can quibble all I want about the clinical trial, but the real world is the real world.’”

What do we know about each vaccine’s ability to protect against the variants?

We know more about how the J&J vaccine protects against the coronavirus variants because the trials were conducted in South Africa and Brazil when the new variants had become prevalent. In South Africa, some 95 percent of circulating virus was the B.1.351 variant and in Brazil, 69 percent of the circulating virus was a P1/P2 variant at the time of the trial. Although the J&J vaccine appeared to be less effective against mild and moderate disease in these regions, it remained strongly protective against severe disease, hospitalizations and deaths.

“We cannot say definitively that the difference in efficacy in these regions is due to more variants, but it’s an assumption,” said Gandhi.

In the U.S., where the prevalence of variants was low, the overall efficacy of 72 percent likely was not affected by the variants, said Gandhi.

In contrast, the mRNA vaccine trials were not conducted in the presence of high levels of the variants, so less is known about how well they protect against the variants. Real-world data from Pfizer vaccinations suggest they are effective against the B.1.1.7 variant originally detected in the U.K.

What do we know about each vaccine’s ability to prevent asymptomatic infection?

We also have more information about asymptomatic infections from the J&J trials because they included asymptomatic PCR testing and antibody testing. (Participants were tested for antibodies to a part of SARS-CoV-2 that was not the spike protein since the vaccine would induce antibodies to the spike protein.) The vaccine was found to be 74 percent effective against asymptomatic infection, similar to its protection against symptomatic infections.

The mRNA vaccine trials did not look for asymptomatic infections, but Gandhi thinks all the vaccines likely offer parallel protection against symptomatic and asymptomatic infections. “They didn’t do this kind of testing, but based on the real-world data from vaccine roll-out, it seems likely your protection against asymptomatic infection will probably mirror your protection against symptomatic infection,” she said.

A study of U.K. health care workers, for example, found that the Pfizer vaccine reduced all infections, including asymptomatic, by 86 percent. A real-world study of the general population in Israel found that the Pfizer vaccine reduced asymptomatic infection by 94 percent. Another study, among pre-surgical patients across the Mayo Clinic system, showed that mRNA vaccines were 80 percent protective against asymptomatic infections.

Were there other differences between the vaccine trials?

Compared to the mRNA vaccine trials, the J&J vaccine trial included more participants who identified as Hispanic/Latino (45.1 percent) and Black or African American (17.2 percent). It also included more participants who were over 60 years old (34.6 percent) and more who had comorbidities (39.9 percent).

The more diverse participants not only help to better determine vaccine efficacy but also “enhances trust among the populations that look like the populations who are enrolled,” said Gandhi.

If you’re somewhat less protected against mild and moderate COVID-19 with the J&J vaccine, does that mean you are more likely to transmit the virus? Should your behavior depend on which vaccine you get?

No vaccine can prevent all COVID-19 infections. To minimize the chances of transmitting the virus even after you’ve been fully vaccinated, Gandhi suggests avoiding contact with unvaccinated people if you experience possible COVID-19 symptoms.

The good news is that vaccination seems to reduce viral load in the small percentage of people who still become infected. Preliminary data from Israel of the Pfizer vaccine suggest that if you catch COVID-19 after vaccination, you’re likely to harbor less virus, making you less likely to transmit.

“I think it’s going to end up being quite hard to transmit virus after you’ve been vaccinated, even if you are exposed, because of the low viral load in your nose,” said Gandhi. “Which is one of the reasons the CDC has said that vaccinated people do not need to quarantine after exposure if asymptomatic. That’s really based on that data.”