7 Things We've All Learned About Vaccines

"There is a lot of optimism because the COVID vaccines have shown us that there are platforms that can develop strong responses in older adults,” Tan says. “I think you're going to see mRNA technology used as a platform for more vaccines.”

3. Their side effects can be — yes — uncomfortable.

For many, the second dose of their COVID-19 packed more punch than they may have expected — by way of fatigue, headache, chills and fever, among other side effects — if only for a day or two.

Reactions from other vaccines are less well-known, but they're not uncommon, experts say. The seasonal flu shot, for example, can cause fever, fatigue and other reactions. And the vaccine to prevent shingles can induce shivering, muscle pain and an upset stomach.

The upside of this kind of discomfort? Side effects are “an indicator the vaccine is working,” Mishori says. “Once [the side effects] go away, you're left with the immune protection.”

4. Protection can be imperfect — yet still lifesaving.

Most vaccines aren't an all-or-nothing proposition. Just because you're vaccinated doesn't mean you won't get sick. However, the vaccine significantly reduces your risk of having a severe case.

"If you do get illness, it will be a milder illness,” says Clare Rock, M.D., an infectious diseases physician and hospital epidemiologist at Johns Hopkins School of Medicine. “The chance of having a serious illness and needing hospitalization is much, much lower.”

Many Americans seem to have a better understanding of vaccine effectiveness as a result of the COVID-19 vaccines. That's important, Tan says, because the numbers can be confusing. For example, if the seasonal flu vaccine is 40 percent effective one year, that doesn't mean that 60 out of 100 vaccinated people will get influenza. What it actually means, he explains, is that vaccinated people are 40 percent less likely to get influenza when compared with people who didn't get the vaccine.

In their clinical trials, both of the mRNA vaccines for COVID-19 posted efficacy rates of about 95 percent — exceeding scientists’ expectations by a long margin. The data shows that the actual percentage of vaccinated people who got COVID-19 in both of those trials was just 0.4 percent, and none died of the disease.

The vaccine's efficacy in preventing infection has likely dropped in recent months, with the delta variant fueling breakthrough cases across the country. But the vaccines are still highly effective when it comes to what's most important: preventing death.

5. They are rigorously tested – group by group by group

As anyone reading headlines in the past year and half knows, all authorized vaccines — even those that received emergency authorization from the Food and Drug Administration (FDA) — go through many rounds of study, examination and research before they are allowed to be used.

Mishori says manufacturers must conduct clinical trials on thousands of people and that vaccines have to show efficacy across different groups: sex, race/ethnicity and age. The data is then examined by a panel of scientific advisers who make recommendations to the FDA and the CDC.

"In general, the CDC can't make a recommendation for an age group unless clinical trials have happened in that age group,” Mishori says, because “there are all kinds of physiological differences.” For example, she says, the kidneys of older people may not be as robust in clearing medications as they are in younger patients — so it's especially important for vaccines to be tested in adults over 65.

Many manufacturers go through the FDA process and never get authorization, Tan says. “A lot of research goes on that ultimately doesn't meet the FDA minimums. The truth of the matter is, only about 1 in 10 make it out.” Even after an approval, both the CDC and FDA continue to track the safety of all licensed vaccines.

6. Yes, their protection can wear off over time.

Studies in recent months reveal that the effectiveness of the COVID-19 vaccines does appear to wane. A Mayo Clinic study from five states, conducted through July, found that the Pfizer-BioNTech vaccine's effectiveness against the delta variant dropped from 76 percent to 42 percent, while the Moderna vaccine's effectiveness went from 86 percent to 76 percent. Another CDC analysis of both vaccines found that among nursing home residents, the effectiveness against infection dropped from about 75 percent to 53 percent between March 1 and Aug. 1.

The data prompted the Biden administration to announce plans for a vaccine "booster" shot. It's not unusual for a vaccine to require a booster, Rock says. Many childhood vaccines require boosters later in life, for example. How long immunity lasts varies by vaccine. Some shots (measles) protect for life, while others (the flu shot) need to be renewed every year.

"The immune response — the body's memory — can wane over time,” Rock says. “The body needs another reminder, in a safe way, how to respond to the virus.”

A vaccine's durability depends on a variety of factors, Rock adds, including the type of vaccine, an individual's immune system and whether mutations in the virus allow it to evade immunity.

7. They work best if both you and the grandkids get them.

Since the pandemic arrived, there has been a lot of talk about so-called “herd immunity” or “community immunity.” This is the idea that if enough people are vaccinated against a pathogen, it can't travel as easily from person to person — and the entire community is less likely to get the disease.

Many experts talk about community immunity as a way to end the COVID-19 pandemic. But it's also how we protect those who can't get vaccinated (like children who are younger than 12 who can't get the COVID-19 vaccine) or those who don't mount a strong immune response to a vaccine due to their age or health condition, Mishori says.

The concept of community immunity also applies to other vaccines such as the TDAP booster, Mishori notes. “When I see older patients who have a new grandkid, I tell them: ‘If you're going to hold the newborn, you need the TDAP booster.’ Newborns aren't able to get it early, and whooping cough [pertussis] is a very severe disease in infants before they can get vaccinated.”

A vaccine is a means of contributing to society as well as to your immediate family, she says. “You are protecting those who can't protect themselves.”