Moderna (MRNA -0.74%) and Pfizer (PFE 0.23%) are two healthcare companies aiming to produce a vaccine for the respiratory syncytial virus (RSV). In trials, their vaccine candidates have been proving highly effective, and it may not be long before these COVID-19 vaccine makers are fighting for market share again -- this time in the RSV market.

But they also won't be alone. The Food and Drug Administration (FDA) recently approved an RSV vaccine, and it wasn't from either of these two companies.

GSK's vaccine is the first to cross the finish line

On May 3, the FDA approved the first vaccine targeting RSV for people 60 and over. GSK (GSK -0.12%) -- formerly known as GlaxoSmithKline -- is the maker of the vaccine, Arexvy, which has been highly effective in trials. It was 94.1% effective in lowering the risk of severe illnesses, and overall efficacy was 82.6%.

The vaccine was also shown to be safe, with many of the most common symptoms involving pain at the injection site, fatigue, myalgia, and headache. At its peak, analysts expect it could generate up to $2.5 billion in revenue for GSK.

More vaccine approvals likely to follow

GSK's RSV vaccine was first, but it probably won't be long before there's another one available. Both Moderna and Pfizer have been working on RSV vaccines of their own, and their approvals could be coming soon. In August 2022, Pfizer reported that its RSV vaccine for older adults had an efficacy rate of 85.7% against severe diseases in phase 3 trials. And earlier this year, Moderna's RSV vaccine was shown to be 83.7% effective against lower respiratory tract disease in late-stage trials.

Looking at efficacy rates isn't an apples-to-apples comparison, and it's not terribly useful to try to compare the different rates as the trials are made up differently and have different endpoints. The key takeaway for investors, however, is that they have all shown a high degree of efficacy. And that means shots from both Pfizer and Moderna are likely to also obtain approval from the FDA, potentially within the next year.

But the RSV market may not be all that big

Getting another vaccine to market can be exciting for investors of these three companies, but in the grand scheme of things, it may not be all that significant. Analysts from Research and Markets project that the global RSV market was worth less than $1.7 billion in 2020. And while it is growing at a compound annual rate of 14.9%, it will still only be worth an estimated $4.2 billion by 2027.

That's not a huge pie for three large healthcare companies to slice up among themselves, especially for Pfizer and Moderna, which are going to see sales nosedive significantly as demand for their COVID-19 vaccines dries up. Moderna, for example, generated more than $18 billion in revenue from its COVID-19 vaccine last year. 

This shouldn't affect Moderna and Pfizer investors

Although Moderna and Pfizer's RSV vaccines are lagging behind GSK, that doesn't drastically impact the investment thesis behind these two healthcare stocks. Both Pfizer and Moderna relied heavily on COVID-19 revenue in recent years, and both have a lot of work to do to make up for lost sales.

Pfizer has been busy with acquisitions, and Moderna has been working on investing in its pipeline; one of its most promising assets is a personal cancer vaccine it is working on with Merck, which has been showing some success.

Ultimately, there's still some risk around Pfizer and Moderna, more so with the latter than the former. That's because Pfizer already had a robust portfolio of products before sales of its COVID-19 vaccine took off. Sales from their respective RSV vaccines should boost revenue in the long run, but I wouldn't expect it to be a game changer for either business as both of them need a lot more than just an RSV vaccine to make up for their declining revenue.

GSK, meanwhile, trading at just 13 times earnings -- and showing more stable earnings and growth -- may be a safer option for long-term investors.