Personalized cancer vaccine launched by Moderna: Drug firm’s shares soar more than 140% — should you buy?

Biotech stocks like Moderna have also tended to fall harder than the broader market during downturns. Across seven major market shocks since 2018, Moderna’s average peak-to-trough decline was about 31%, compared with 17% for the S&P 500 over the same stretch, and its single worst drop reached 50%. None of that means the cancer vaccine data isn’t meaningful. It means the stock’s price can move sharply on hope, and just as sharply on second thoughts, long before any of these treatments reach the market.

What this means if you hold US stocks in a TFSA, RRSP or regular account

For Canadians who already own, or are tempted to buy, Moderna or any U.S.-listed stock, the excitement of a spike comes with a few practical wrinkles. The shares trade in U.S. dollars, so a Canadian investor’s return depends on the exchange rate as well as the stock price. And where the investment is held changes the tax math if it’s eventually sold for a profit.

Inside a Tax-Free Savings Account (TFSA), a capital gain is tax-free — but so is a capital loss, meaning it can’t be used to offset other income. Inside a Registered Retirement Savings Plan (RRSP), gains grow tax-deferred until withdrawal, when they’re taxed as regular income. In a non-registered account, only 50% of a capital gain is added to taxable income, a rate the federal government confirmed in 2025 would not rise to 66.67% as it had once proposed. That 50% inclusion rate applies whether a stock is up 10% or 177% in a day — the tax bill only shows up once the shares are actually sold.

Then there’s the U.S. withholding tax. While no withholding tax applies to capital gains for Canadian investors on U.S.-listed stocks held in any type of account, the same does apply to dividends paid out on those U.S. shares. If held inside an RRSP, those U.S. dividends are exempt from the 15% withholding tax, but those same dividends are taxed inside a TFSA.

How to think about a stock spike without chasing it

Before buying into a stock after a headline-grabbing jump, a few questions can help:

• What’s actually been proven and what still needs to be? Remember that early trial data isn’t a drug approval.
• How much of a portfolio would this one stock represent? Concentrating savings in a single volatile name works against the diversification that index funds are built to provide.
• Would a 13% overnight drop be tolerable? With a stock this volatile, that kind of move can happen before the market even opens.
• Is this a bet on the company or a reaction to the headline?

Bottom line

A stock that doubles in a day can be a genuine signal that something important happened, or it can become a magnet for investors chasing a headline instead of a plan. Before adding a name like Moderna to an RRSP, TFSA or any other account, the more useful question isn’t whether the stock will keep climbing; it’s what role a single, high-volatility stock is meant to play in a portfolio built to last longer than one news cycle.