Johnson & Johnson's single-dose COVID-19 vaccine has an overall efficacy of 66%, the company said Friday after a huge multi-continent trial, but the shot does not protect as well against a variant first detected in South Africa.
Encouragingly, the vaccine was able to prevent 85% of severe COVID-19 cases across all geographical regions however -- a key indicator lauded by experts.
Its effectiveness against all forms of the disease -- mild, moderate, and severe -- was as high as 72% in the United States, but fell to 57% in South Africa, where a more transmissible new variant is dominant. In Latin America, it was 66% effective.
"We're proud to have reached this critical milestone and our commitment to address this global health crisis continues with urgency for everyone, everywhere," the company's CEO Alex Gorsky said.
White House spokeswoman Jen Psaki said President Joe Biden was "encouraged" by the data and the Food and Drug Administration would carry out its own evaluation.
J&J said it would apply for US emergency authorization in early February, and the vaccine will then likely become the third available in the world's hardest-hit country.
The Pfizer and Moderna vaccines were the first to be authorized in the US, and both have efficacies of around 95%.
While the J&J vaccine is not as protective, the comparisons are not quite like-for-like, because those trials reported results before newer, more transmissible mutations of the virus became dominant in some parts of the world.
These variants, such as B.1.135 in South Africa and P.1 in Brazil, elude some of the blocking action of antibodies triggered by vaccines made against the common strain of the coronavirus.
But the J&J vaccine requires only one shot and can be stored for up to three months at 36-46 Fahrenheit (2-8 degrees Celsius), giving it a logistical edge over the Pfizer and Moderna vaccines.
"The fact we have a single-dose vaccine that's 66% effective, and highly effective against severe disease, is still a success," said Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, adding it would increase US vaccination capacity.
"The real headline result is that a single shot vaccine, capable of easy long-term storage and administration provided complete protection against hospitalization and death," added Kevin Marsh, a professor of tropical medicine at the University of Oxford.
J&J says that if authorized it is on track to deliver 100 million doses to the US by the end of June.
Viral vector vaccine
The J&J shot uses a genetically-engineered version of a common-cold causing adenovirus as a "vector" to shuttle genetic instructions into human cells, telling them to create a protein of the coronavirus.
This in turn evokes an immune response, and the same technology -- known as an adenovirus vector vaccine -- has been used in the company's European Commission-approved Ebola vaccine.
The J&J vaccine uses double-stranded DNA molecules to deliver its genetic instructions, which are hardier than the single-stranded RNA molecules used in Moderna and Pfizer vaccines, and eases the cold storage requirements.
Vector vaccines also have downsides: the human body can develop an immune response to the vector, attacking it and reducing the efficacy, particularly of booster shots.
This was a weakness of the AstraZeneca vaccine, which uses the same technology but comes in two doses.
The Phase 3 J&J trial was conducted in eight countries, involving 43,783 people, 34 percent of whom were over 60.
This broke down to 19,302 in the US, 17,905 in Latin America, and 6,575 in South Africa.
The differences in its efficacy across these regions further emphasizes the growing threat posed by new variants.
Anthony Fauci, the US top infectious disease official, said it highlighted the need to step up the vaccination rate quickly before new variants establish themselves firmly in the US, and to quickly develop new vaccines directed at variants.
"The fundamental principle of getting people vaccinated as quickly and as efficiently as you possibly can will always be the best way to prevent the further evolution of any mutant," he told reporters.