Are COVID-19 Vaccines Safe and Effective?

Five years after Donald Trump declared a national COVID-19 emergency, here's what the research says.

"We are emerging from one of the darkest years in our nation's history into a summer of hope and joy, hopefully," declared President Joe Biden at a press conference on July 6, 2021. "We're closer than ever to declaring our independence from this deadly virus." The cause for his optimism was the fact that more than 182 million Americans had received at least one shot of the new COVID-19 vaccines, including nearly 90 percent of seniors and 70 percent of adults over the age of 27.

That 70 percent figure was significant because early in the pandemic many epidemiologists had suggested that was immunological threshold at which a population might achieve herd immunity. Herd immunity occurs when a sufficiently large portion of a population is immune to a disease either via vaccination or infection, making it difficult for the disease to continue to spread.

Biden did cautiously note the emergence of the new more highly infectious Delta variant of the coronavirus but asserted that "the good news is that our vaccinations are highly effective." He added, "If you're vaccinated, you're protected."

Biden was speaking nearly a year and a half after his predecessor, Donald Trump, had declared a national state of emergency over the novel coronavirus outbreak on March 13, 2020. Three days later, Trump's White House issued the President's Coronavirus Guidelines for America that, among other things, advised governors in states with evidence of community transmission to close schools, bars, restaurants, food courts, gyms, and other indoor and outdoor venues. On May 15, 2020, Trump launched Operation Warp Speed to rapidly produce COVID-19 vaccines. Relying on the amazing success of Operation Warp Speed, Biden was calling for the end of pandemic lockdowns and hailing the advent of a "summer of freedom."

In earlier articles on the fifth anniversary of Trump's national emergency, we have considered whether face masks worked, the effectiveness of hydroxychloroquine and ivermectin as COVID-19 treatments, and how many Americans died of the infection. Sticking to recent peer-reviewed science and setting aside the political question of what the government should do with the information, let's turn now to the question: What have researchers learned about the efficacy and safety of COVID-19 vaccines?

Initial phase III trial results from November 2020 suggested that, after two doses, new mRNA vaccines developed by Moderna and Pfizer were about 95 percent effective in protecting against infection from COVID-19. A March 2021 real-world study involving health care, first responders, and essential workers bolstered those findings.

Herd Immunity

Just as the national inoculation campaign began in December 2020, the director of the National Institute of Allergy and Infectious Diseases (NIAID), Dr. Anthony Fauci, was predicting that the U.S. could achieve herd immunity by "the end of the second quarter of 2021."

Achieving herd immunity depends upon vaccines and natural infection creating durable long-term protection against reinfection (e.g., measles, smallpox, and polio). The clinical trials showed that the initial protection afforded by the new COVID-19 vaccines was outstanding but being short-term the trials did not have the power to determine how long that protection against infection might last.

Early on, some other researchers questioned the possibility of achieving COVID-19 herd immunity via vaccination and/or natural infection. In a September 2020 preprint(subsequently published in Nature in January 2021), a team of Australian immunologists sought to determine how COVID-19 immunity might evolve. Noting that protective antibodies waned in the first 2–3 months following infection by four known common cold coronaviruses, they suspected that that would also be the case for the novel COVID-19 coronavirus. "Our study suggests SARS-CoV-2 immunity after infection is likely to be transiently protective at a population level," they concluded.

An April 2021 article in PLoS Pathogens by an immunologist at the NIAID similarly concluded from experience with the serial reinfection by four common cold coronaviruses that "COVID-19 herd immunity is a pipe dream." (Recent research speculates that the Russian Flu pandemic of the late 19th century might actually have been caused by an outbreak of one the now endemic common cold coronaviruses.)

By March 2022, Fauci and his colleagues acknowledged "the concept of classical herd immunity may not apply to COVID-19" in The Journal of Infectious Diseases. "Living with COVID-19 is best considered not as reaching a numerical threshold of immunity, but as optimizing population protection without prohibitive restrictions on our daily lives," they concluded. Population protection, among other things, now involves inoculations updated much like seasonal flu vaccines to boost waning antibodies and to counter emerging variants of the COVID-19 coronavirus. Ultimately, Biden's 2021 "summer of freedom" turns out to have been a fond but illusory hope of a permanent respite from COVID-19. Given this reality, current COVID-19 vaccines are now primarily designed to prevent severe disease and death rather than infection.

Lives Saved

COVID-19 vaccines and boosters have proved to be highly effective in preventing severe cases, hospitalizations, and deaths. Recent research in The Lancet calculates that COVID-19 vaccinations between December 2020 and March 2023 saved approximately 1.6 million lives in Europe. A 2024 Brookings Institution report suggests "the delivery of vaccines to a substantial majority of the American population by mid-2021 saved close to 800,000 American lives relative to what would have occurred had vaccines not been developed." In 2023, a team of researchers associated with Harvard University and the Centers for Disease Control and Prevention distressingly estimated that "at least 232,000 deaths" in the U.S. "could have been prevented among unvaccinated adults during the 15 months [May 30, 2021 to September 3, 2022] had they been vaccinated with at least a primary series."

Safety and Side Effects

Let's focus on the mRNA vaccines developed by Moderna and Pfizer since they account for the vast majority of COVID-19 inoculations in the United States. In reporting the results from their Phase III clinical trials in November 2020, both Pfizer and Moderna chronicled relatively few transient side effects, which were mostly mild and moderate. These included fatigue, headache, and pain at the injection site.

As reassuring as those results were, the rollout of vaccines to tens of millions of people would likely uncover other side effects. In the spring of 2021, women began reporting prolongation of their menstrual cycles after receiving mRNA vaccinations. Subsequent studies confirmed this side effect but concluded that it was temporary and had no identifiable effect on fertility.

As the number of vaccinations increased, reports emerged in the summer of 2021 that after getting their second mRNA inoculation, several young males had experienced a type of inflammation of the heart called myocarditis. Understandably, parents found these reports alarming. Fortunately, subsequent research has been more reassuring.

A September 2024 review in the journal NPJ Vaccines reports that the risk of myocarditis is about six times greater for those who are infected with COVID-19 than for those who are vaccinated. A February 2025 article in the European Heart Journal compared patients who experienced post-vaccine myocarditis to those who experienced post-COVID-19 and conventional myocarditis. The researchers found that post-vaccine myocarditis patients were less likely to be hospitalized and experienced fewer cardiovascular events.

In January 2023, Fox News presenter Tucker Carlson asserted that a number of young athletes were dropping dead shortly after getting COVID-19 injections. Carlson claimed, "Since the vax campaign began, there have been more than 1,500 total cardiac arrests in those [European sports] leagues and two-thirds of those were fatal," reported the Associated Press.

This claim was widely debunked shortly thereafter. More recently, a November 2023 study in Sports Health of 1,229 vaccinated U.S. Olympic athletes found that none had died of sudden cardiac arrest or experienced myocarditis. A February 2025 cardiology research letter in JAMA Network compares sudden cardiac arrest and death rates among young competitive athletes before and during the pandemic. "This cohort study found no increase in SCA/SCD [sudden cardiac arrest/sudden cardiac death] in young competitive athletes in the US during the COVID-19 pandemic, suggesting that reports asserting otherwise were overestimating the cardiovascular risk of COVID-19 infection, vaccination, and myocarditis," conclude the authors.

What does recent research tell us about COVID-19 vaccine safety more generally?

The most comprehensive analysis of the safety of COVID-19 vaccines is the cohort study of 99 million vaccinated individuals published in April 2024 in Vaccine. The researchers confirmed that the incidences of previously identified rare safety signals following COVID-19 vaccination were quite low. "What we take away, is that the Covid-19 vaccination campaigns have been very effective in preventing severe disease," explained study co-author epidemiologist Anders Hviid to SciCheck. "The few serious side effects that we have observed in this and other studies have been rare."

A September 2024 analysis in Vaccine of the U.S. COVID-19 vaccine safety surveillance system bolstered this conclusion and found that "the comprehensive federal vaccine monitoring efforts have provided robust data supporting the safety of COVID-19 vaccines. After the administration and monitoring of more than 676 million doses in U.S. residents, serious AEs [adverse events] were rarely observed following vaccination."

Upshot: The promise of vaccinated COVID-19 herd immunity has proved illusory. The coronavirus is now an endemic respiratory illness, much like seasonal influenza. The good news is that the benefits of COVID-19 vaccines considerably outweigh their risks.