The study said the side effects included "pain, feeling feverish, chills, muscle ache, headache, and malaise." That doesn't sound minor, especially when you see that participants were given a gram of acetaminophen every six hours for 24 hours after the injection of the vaccine to reduce fever, headaches, or muscle aches. Of course, given a choice between getting COVID-19 or dealing with a headache and a little fever, most people would gladly choose the latter. However, the fact that the side effects were largely glossed over made some wonder whether researchers are being overly optimistic about the vaccine's outcome.

To be fair, other recent COVID-19 vaccine candidates have shown similar side effects. Nearly all phase 2 test subjects given mRNA-1273, a candidate from Moderna (NASDAQ:MRNA), exhibited fatigue, fever, muscle aches, and headache.

Another, arguably larger reason for the drop in AstraZeneca's stock is that the pharmaceutical company is competing against so many other vaccine candidates. Partners BioNTech (NASDAQ:BNTX) and Pfizer (NYSE:PFE), for instance, also announced phase 1/2 clinical results for their vaccine candidate on Monday, though they did so via a preprint (without peer review) rather than in a peer-reviewed journal. Moderna's most recent phase 1/2 results came out last week in a New England Journal of Medicine report. 

Still a good stock, with or without a vaccine

Vaccines normally take up to 10 to 15 years to produce, not months. Things are rushing forward at light speed because of the worldwide necessity for a coronavirus vaccine, but it's equally possible that multiple candidates will be successful or that none of them will work in the real world. Despite our best efforts, we still don't have vaccines for malaria, HIV, or tuberculosis. 

Another potential concern is that while AZD1222 has been shown to produce antibodies, it hasn't been proven yet whether those antibodies will prevent the virus from hitting healthy cells -- or, if so, for how long. The vaccine works by mimicking the spike protein used by COVID-19 to infect cells. The hope is that after being vaccinated, the body's immune system will recognize the spike protein and swiftly mount its defense.

More encouraging, perhaps, is that the vaccine elicited a T-cell response, which should stimulate the immune system to attack cells infected by the virus.