New results on a possible COVID-19 vaccine from Oxford University and drugmaker AstraZeneca suggest it is safe and about 70% effective. Some experts say that shows it is likely to win approval. But questions remain about how well it may help protect those over 55. That’s a key concern for a vaccine that health officials hope to rely on around the world because of its low cost, availability and ease of use. Partial results were published Tuesday by the medical journal Lancet.