With Moderna's (MRNA -0.24%) mRNA vaccine in high demand as COVID-19 cases spike, the price of Moderna stock has been on an upward swing. It's now trading at almost $400 per share, and there's plenty of potential for further growth with more promising vaccines under development -- including a combined flu and coronavirus shot. 

If you were hoping to invest in Moderna, though, you may be disappointed to find that buying even a single share would set you back close to $400.

The good news is, you don't need that much cash in a brokerage account just to own a piece of this popular pharmaceutical company. In fact, you can buy in for the cost of a penny stock. Here's how. 

Investor reviewing charts.

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Almost anyone can afford Moderna shares using this approach

Although Moderna shares cost around $382.22 as of Aug. 28, 2021, it's possible to buy into the company for pennies. That's because there's no longer a requirement that you purchase a full share.

See, many brokerage firms now allow investors to purchase something called fractional shares. These are parts of shares. Partial shares have always been available, with investors sometimes receiving them after stock splits. But brokers only recently began allowing people to purchase them intentionally by specifying the dollar amount they want to invest in a stock. 

Some brokerage firms have minimums you must invest to buy fractional shares, such as buying at least $5 worth. But others don't. In fact, with some brokers, it may be possible to invest with as little as a cent as long as you're buying at least 0.001 of a share.

Because fractional shares allow you to decide how much money to invest, rather than the number of shares to buy, you can purchase Moderna stock if you have very little money. If you have only a dollar to spend, you'd get 0.002 of a share. But the right broker would allow that, and Moderna would become part of your portfolio. 

You may be wondering if it's worth buying such a small amount of a stock. But, since most brokers have eliminated commissions, you won't pay a fee so there's little downside to getting started investing with whatever amount of money you have available. And if Moderna's stock price goes up, the value of your investment increases by the same percentage as people with full shares. 

Fractional shares and $0 commissions make it easy to add to your investment balance over time. So while you may start with 0.002 of a share, you could buy another $1 worth of stock the next week, and the next, and so on. Eventually, you could end up with a valuable portfolio -- especially if the stock price rises. 

Whether you should add Moderna or any other stock to your portfolio does depend on whether you believe in the long-term viability of the company, though. After all, even if you aren't spending a lot of money, you don't want to lose what you have.

But the good news is, there is a lot more information about Moderna available than there is data about the many untested pharmaceutical companies offering penny stocks for sale. So you can make a fully informed choice about what's right for you and, if you decide Moderna is an investment you want to make, you have the option to add it to your portfolio for the cost of those penny stocks.