The stock market has performed extremely well lately, with several major market benchmarks rising to record highs in the past few weeks. A big portion of the enthusiasm for stocks lately has come from pharmaceutical and biotech companies coming out with various vaccine and treatment candidates to fight against the COVID-19 pandemic.

Given how important finding an end to the coronavirus crisis is for the entire world, it's not surprising that investors have celebrated the companies that have managed to deliver promising treatments and vaccines. But many market participants wonder how much more upside there could be for markets once vaccines become readily available and treatments start to work more effectively. At some point, the stock market could start losing its upward momentum even as more good news surfaces.

Hand with blue glove holding dropper putting blue liquid into tubes, with box labeled COVID-19 on lab table with other supplies.

Image source: Getty Images.

How markets are faring Monday morning

Stock markets jumped out to early gains. But by mid-morning, most major benchmarks had given up their gains, and some headed lower.

As of 11:30 a.m. EST, the Dow Jones Industrial Average (DJINDICES:^DJI) was still up 150 points to 29,414. However, the S&P 500 (SNPINDEX:^GSPC) was up just a single point to 3,559, and the Nasdaq Composite (NASDAQINDEX:^IXIC) was down 39 points to 11,816.

Good news from AstraZeneca -- but not for its shareholders

The latest news on the COVID-19 front came from the U.K. and Brazil. AstraZeneca (NASDAQ:AZN) has been working with the University of Oxford to come up with a potential coronavirus vaccine, and the latest interim analysis with participants in those two countries showed promising results from the candidate.

The early results for the AZD1222 vaccine were encouraging, although perhaps not as much so as previous efforts from Pfizer (NYSE:PFE) and Moderna (NASDAQ:MRNA). AstraZeneca said that average efficacy for the vaccine candidate was 70%. However, what made headlines was that one particular dosage regimen showed results of roughly 90% effectiveness.

The most successful dosage regimen involves a smaller dose early followed by a larger dose about a month later. AstraZeneca CEO Pascal Soriot said that the fact that a smaller early dose is effective is good news, as it'll let more people get started with a course of vaccination as limited supplies of the vaccine become available. AZD1222 is also easier to store than some of its rivals, giving it a potential competitive advantage.

Nevertheless, AstraZeneca stock fell more than 3% on Monday morning. Pfizer's stock was down 1%, while Moderna picked up 2% on the day.

Vaccine fatigue?

Stock market prices are forward-looking, and they tend to move the most when investors get new and unexpected information. When the first successful vaccine news came out, stocks roared higher. As a result, current prices already reflected the likelihood of at least one vaccine becoming available soon. AstraZeneca's news didn't really change that assessment, instead simply providing redundant safeguards in case another vaccine candidate proves not to be as effective as anticipated.

Even news of a possible treatment for COVID-19 didn't get a huge upward move from shareholders. Biotech stock Regeneron Pharmaceuticals (NASDAQ:REGN) got Emergency Use Approval for its antibody cocktail treatment, yet the stock was up only a fraction of a percent.

Stay-at-home stocks have been volatile amid news of a possible end to the health crisis, and investors need to anticipate that further news on the coronavirus vaccine and treatment front could produce unexpected moves in the stocks involved. It's no longer a foregone conclusion that a stock will soar on good COVID-19 news, or that the whole market will share in a single company's success. Shareholders need to act accordingly.