A recent study from Comparitech shows that share prices for large breached companies will hit a low point approximately 14 market days after an incident becomes public. Share prices fall 7.27 percent on average to reach that low, and they underperform the NASDAQ by -4.18 percent. Finance and payment companies saw the largest drop in share price performance following a breach, while healthcare companies were least affected. Social media and e-commerce companies had the worst long-term effects on share price on average. The study analyzed stock performance for 28 very large companies listed on the New York Stock Exchange.
Source: https://threatpost.com/data-breaches-batter-stock-price/150088/