(Corrects typographical error in first paragraph) * J&J COVID-19 vaccine 72% effective in U.S., 66% globally * GameStop plays surge as brokerages ease restrictions * Popular mega-cap long positions take a hit * Eli Lilly gains as Q4 profit rises 41.5% * Futures down: Dow 0.94%, S&P 0.81%, Nasdaq 0.88% Jan 29 (Reuters) - U.S. stock indexes were set to open lower on Friday after COVID-19 vaccine data from Johnson & Johnson hurt sentiment and added to worries over a growing standoff between hedge funds and retail investors. Shares of Johnson & Johnson fell 3.9% in premarket trading after the drugmaker said its single-dose vaccine was 72% effective in preventing COVID-19 in the United States, with a lower rate of 66% observed globally. Those results compare to the high bar set by two authorized vaccines from Pfizer Inc/BioNTech SE and Moderna Inc, which were around 95% effective in preventing symptomatic illness in pivotal trials when given in two doses. “While it’s good to have another entrant, the question is the efficacy. The concern is if it’s a lot less effective, then investor and consumer confidence will be substantially lower,” said Sam Stovall, chief investment strategist at CFRA Research. The S&P 500 and the Nasdaq indexes were set for their worst weekly performance in two weeks, while the blue-chip Dow tracked its biggest weekly fall since the end of October. Worries of a short squeeze grew after an army of retail investors returned to trade shares in companies including GameStop Corp, AMC Entertainment Holdings Inc and BlackBerry Ltd. Shares of the companies surged after Robinhood and Interactive Brokers said they planned to ease restrictions after imposing buying halts a day earlier. Investor favorites including Apple Inc were sold off by hedge funds recently to cover billions of dollars in losses. Shares in Apple, Amazon.com Inc, Microsoft Corp , Facebook Inc, Netflix Inc, Tesla Inc and Alphabet Inc declined between 0.5% and 1.2%. “The markets were vulnerable to a decline because of how far they had advanced as compared with either moving averages, and this Reddit activity was the catalyst that sort of triggered the sell off,” Stovall said. Better-than-expected quarterly earnings reports and hopes of a speedy economic recovery under the Biden Administration, on the back of massive fiscal and monetary stimulus program have helped Wall Street’s main indexes trade at record levels recently. However, concerns over stretched valuations, rising coronavirus cases and new variants of the virus have kept investors on edge about a pullback and an increase in volatility in the near-term. The first known U.S. cases of the South African COVID-19 variant, found to be partly resistant to current vaccines and antibody treatments, was detected in two South Carolina patients on Thursday. At 8:29 a.m. ET, Dow e-minis were down 288 points, or 0.94%, S&P 500 e-minis were down 30.75 points, or 0.81%, and Nasdaq 100 e-minis were down 116 points, or 0.88%. Declines in stock index futures were tempered on hopes of more stimulus as data showed U.S. consumer spending fell for a second straight month in December. Eli Lilly and Co jumped 4.2% after the company posted a 41.5% rise in fourth-quarter profit, on higher demand for its diabetes drugs and a successful launch of its COVID-19 antibody treatment.