When it rains, it pours. Instead of a slow slide, COVID-19 vaccine sales at Moderna (MRNA 1.58%) collapsed in the third quarter. 

Investors who were hoping for steady cash flows from this vaccine maker were deeply disappointed when it reported results on Thursday, Nov. 3, 2022. Is it time to cut and run, or are there better days ahead? Here's what you should know.

A steep decline

Moderna has been plowing COVID-vaccine profits into its development pipeline. This could set the company up with new revenue streams, but for now, this is still a one-product business. 

Unfortunately, third-quarter sales of Moderna's only product fell 35% compared to last year. This is an especially bad time to lose revenue. Developing a larger slate of experimental new drugs pushed Moderna's operating expenses 56% higher year over year.

Lowered sales combined with rising expenses have been disastrous for Moderna's bottom line. Third-quarter net income fell 69% year over year.

Bottom line losses ahead?

Moderna was able to squeak out a $1.04 billion profit in the third quarter. Sadly, this might be the last time we see positive cash flows from Moderna for a while.

The FDA authorized new omicron-targeting booster shots from both Moderna and Pfizer (PFE 0.66%) on Aug. 31, but uptake has been tepid at best. On Sep. 30, a survey from the Kaiser Family Foundation found that less than one-third of American adults had been injected with a new booster or intended to do so.

In addition to declining revenue, investors can expect rapidly rising operating expenses from Moderna in the quarters ahead. This September, management boasted that it had 24 vaccines in clinical trials. Three years earlier, there were just eight.

No two drug development programs are identical, but they all share a common feature. They get more expensive as they get closer to the finish line. Now that the company has four late-stage programs, research and development expenses could push Moderna's bottom line back into the red.

Reasons to buy Moderna stock right now

Moderna probably won't be a one-hit wonder for much longer. In the first quarter of 2023, the company expects to read out results from a phase 3 trial with an experimental new flu vaccine tentatively named mRNA-1010.

This winter, the company could also read out results from a phase 3 trial with over 35,500 older adults and mRNA-1345. This is a candidate for protection against respiratory syncytial virus (RSV). This virus is dangerous for at-risk patients, but it spreads easily among relatively healthy people who hardly know they have it. It's also a leading cause of hospitalizations among infants and older adults.

Despite multiple past attempts that have come close, there still aren't any approved vaccines proven to protect against RSV infections. If mRNA-1345 becomes the first, it could quickly begin generating more than $1 billion in annual revenue for Moderna.

RSV isn't the only unvaccinated disease Moderna is going after. The company also boasts a candidate for cytomegalovirus (CMV) that's in an ongoing phase 3 trial.

Probably not a buy right now

You can buy shares of Moderna for just 4.5 times trailing earnings right now. That may seem like a bargain, but it's important to remember that success for Moderna's non-COVID-related pipeline is a long way from guaranteed.

Pfizer is also developing an RSV vaccine candidate, and the larger drugmaker already has some successful phase 3 trial results. If Moderna's RSV vaccine outperforms Pfizer's, it will still need to contend with one from GlaxoSmithKline (GSK -0.07%) that is even further ahead on its development timeline. The FDA is currently reviewing an application for GlaxoSmithKline's RSV vaccine, and a positive approval decision is expected on or before May 4, 2023.

At such a low valuation, it probably makes sense to hang on to any Moderna shares that you might be holding. However, with declining COVID-19 vaccine sales and no certainty that there will be new blockbuster drugs to pick up the slack, buying the stock doesn't look like a smart move right now.