How should you interpret vaccine efficacy—what does “95 percent efficacy” or “66 percent efficacy” even mean?
Vaccine efficacy refers to how well it performs in a carefully controlled trial, whereas effectiveness describes its performance in the real world. A common mistake is to interpret a 95 percent efficacy to mean you have a 5 percent chance of getting sick even if you get vaccinated. That is not true at all. Efficacy is calculated based on trials that have an unvaccinated placebo control group, and at the end of the trial, they look at the number in the control group that ended up with symptomatic COVID to get the baseline infection rate. The Pfizer-BioNTech vaccine, for instance, had 95 percent efficacy in its clinical trial. That number came from the fact that 162 people in the placebo group got symptomatic COVID, and eight people in the vaccinated group did; that is 170 cases total. Eight is approximately five percent of 170. That is essentially how efficacy is calculated. Most COVID vaccine clinical trials have reported efficacy in preventing any symptomatic disease, not necessarily severe disease; early data for the latter are promising, however.
If you get the vaccine and still get infected, does the vaccine still make a difference? Will it prevent severe disease or death?
Yes, what we’ve seen with most of the current vaccines is less severe symptoms and almost no hospitalizations and deaths in vaccinated people who become infected. Several vaccine trials have observed a reduction in severe disease in the vaccinated subjects. With a few exceptions—such as, possibly, the AstraZeneca vaccine in South Africa—the vaccines appear to reduce your risk of getting symptomatic COVID, but if you do get it, it’s very likely to be a less severe case.
Do the vaccines protect against the new virus variants, including those first identified in the U.K., South Africa and Brazil?
The data so far suggest that most of the vaccines do provide at least some protection against the new variants. We can see from some of the clinical trial data and antibody neutralization studies, however, that vaccines designed against the original strains of the virus might not be quite as effective against certain new variants—particularly against mutations found in the B.1.351 variant that is now widespread in South Africa. Many of the new variants have changes in the spike protein, which the virus uses to infect cells, so it’s a little bit different than what a vaccinated person’s body has seen before in the vaccine. Still, overall, the vaccines authorized in the U.S. are meeting efficacy targets and seem to give some protection against the new variants. The best way to prevent new variants from emerging is by getting lots of people vaccinated. Moderna is developing a booster shot to offer additional protection if needed, and Pfizer-BioNTech is exploring this possibility as well.
What is the difference between mRNA and viral vector vaccines?
An mRNA vaccine—such as the ones made by Pfizer-BioNTech or Moderna—provides genetic instructions for making viral proteins in the form of single-stranded RNA, in a lipid coat, to your cells, without injecting any virus itself. This has some exciting advantages because it only contains those virus components that you want the immune system to recognize, and it causes your own cells to mimic the way viral proteins are made when a cell is naturally infected with the virus. It is remarkably good at inducing the same type of immune response that would occur in a natural infection. A downside is that the mRNA needs to be kept very cold for it to be stable.
Viral vector vaccines—such as those made by AstraZeneca or Johnson & Johnson—use double-stranded DNA instead of RNA, but are still delivering genetic instructions to your cells. They use a modified, safe virus (not SARS-CoV-2) to deliver the instructions for making viral proteins.
Is there a “best” vaccine?
Different vaccine designs are chosen because they have unique advantages, so there is not a “best” vaccine, but there might be a vaccine that is better for a certain group of people to take. Both mRNA and viral vector vaccines instruct your cells to make pieces of the coronavirus spike protein so that the next time your body encounters those proteins it can mount an immune response. Efficacy numbers are derived from different clinical studies that were done in different settings, so instead of comparing those numbers, the thing to take away is that the authorized vaccines generally work. They have efficacy.
Will the vaccine protect you from giving the virus to others?
We hope so, but we don’t fully know yet. There are two key goals of vaccines: to protect the individual and to stop the spread of disease. We do have reason to be hopeful based on what we know about viruses and some emerging data, but we still don’t know for sure if vaccinations will reduce disease transmission. That is why it’s important to continue social distancing, wearing masks and following other guidelines to prevent the spread of disease, even if you have been vaccinated.
Will the vaccine’s effectiveness “wear off” over time?
This could happen. Vaccines often need booster doses to confer the best levels of protection. Some vaccines can protect someone for a lifetime whereas others require a boost every few years. It will be important to monitor immune responses over time in those who are vaccinated so we will know if and when boosters are needed. Most of the currently available COVID vaccines require two doses to be fully effective. The CDC recently expanded the recommended allowable interval between doses because of limited availability, although some experts have criticized the decision.
Should you be worried about allergic reactions to the vaccine?
Allergic reactions can occur in response to any injected vaccines or drugs, but they are extremely rare. The vaccines should always be administered in a setting where health care providers can treat any allergic reactions that develop unexpectedly. People don’t need to be concerned about the vaccines if they have food allergies or latex allergies. Most people don’t need to worry about an allergic reaction. For those with a history of severe allergic reactions or allergies to the vaccine ingredients, it would be best to discuss it with a doctor before getting the COVID vaccine.
What side effects might you experience, and are those normal?
The common side effects are what we would expect for other vaccines: fatigue, headache, fever—particularly after the second dose—and acute pain at the injection site, which can feel warm to the touch and swollen; all of these symptoms are signs of immune reaction. These reactions are very common and often indicate that the immune system is working hard to recognize the vaccine and remember the new virus for next time.
How can you ensure you’re getting a legitimate, authorized vaccine?
It is always important to take vaccines under the supervision of a licensed medical provider. Make an appointment with a designated clinic or retail pharmacy where vaccines are available. Check with your doctor about any concerns because there are clear distribution plans for vaccines in most places.