By Peter Loftus and Matt Grossman

Johnson & Johnson said Thursday it plans to begin the first human studies of its experimental coronavirus vaccine next week, as it races to make the shot available starting early next year.

The New Brunswick, N.J., company's initial study will aim to enroll more than 1,000 healthy adults, starting first in Belgium on July 22 and then the following week in the U.S. Researchers will assess the vaccine's safety and ability to induce an immune response, J&J Chief Scientific Officer Paul Stoffels said Thursday on a conference call with analysts. The company also plans studies in additional countries.

The more definitive testing will take place as soon as September. J&J said it is still having discussions with the U.S. National Institutes of Health for a large phase 3 clinical trial that could begin in late September, which would test whether the vaccine protects people from Covid-19, the disease caused by the new coronavirus.

J&J could get an answer about whether the vaccine safely prevents Covid-19 by the end of the year, Dr. Stoffels said. If successful, the company expects the shot to become available in early 2021, and J&J plans to manufacture up to one billion doses by the end of next year.

"We have the manufacturing sites, we have the filling sites, we have everything that's needed to deliver that one billion vaccines" next year, Dr. Stoffels said.

The NIH is leading other large, decisive trials for coronavirus vaccines getting under way this summer, including one from Moderna Inc. and another from the University of Oxford and AstraZeneca PLC. More than 160 coronavirus vaccines are in development world-wide, including more than 20 that have started human testing, according to the World Health Organization.

J&J disclosed its vaccine plans while reporting second-quarter financial results that were weighed down by the health and economic effects of the global pandemic. J&J said revenue declined in the latest quarter as the coronavirus pandemic weighed on the company's sales of beauty products and medical devices.

J&J said delays in certain medical procedures, such as knee replacements, hurt its medical-device sales during the quarter, though there were signs of recovery by the end of the period as many doctors and hospitals have resumed such procedures.

A recent surge in the spread of the virus in certain U.S. states, however, threatens new delays in procedures that could pressure sales. J&J said it is monitoring the trends in new cases, hospitalizations and deaths.

"The next few weeks are going to be critical," J&J Chief Executive Alex Gorsky said on a conference call with analysts.

But J&J beat analysts' revenue-and-profit expectations for the second quarter and raised its financial guidance for the remainder of the year.

J&J's second-quarter revenue was $18.34 billion, a 10.8% decline from the $20.56 billion it recorded during the April-to-June period last year but topping Wall Street analysts' consensus estimate of $17.61 billion, according to FactSet.

J&J's second-quarter profit declined 35.3% to $3.63 billion, or $1.36 a share, from $5.61 billion, or $2.08 a share, in last year's second quarter.

Adjusted earnings, excluding certain items, were $1.67 a share, compared with the $1.49 a share that analysts polled by FactSet had expected.

Among J&J's divisions, the medical-devices business was most severely affected by reduced demand during the Covid-19 pandemic. Sales of J&J's devices, which are also used in surgery, declined 33.9% as people delayed nonessential medical care.

Consumer-health sales declined at a more modest rate, falling 7% from last year's second-quarter result. Demand dropped for skin-health and beauty-care products, J&J said, as people stayed home from work and social events. The decline was partially offset by higher sales of the company's over-the-counter medical products such as Tylenol and Listerine mouthwash.

J&J's pharmaceuticals unit posted a 2.1% increase in quarterly sales, helped by gains for several cancer drugs.

The company's outlook for the remainder of the year improved. J&J now expects to record full-year sales of $79.9 billion to $81.4 billion, higher than a prior forecast of $77.5 billion to $80.5 billion. Analysts had forecast full-year revenue of $79.77 billion.

Write to Matt Grossman at matt.grossman@wsj.com