COVID-19 vaccines remain effective at preventing serious illness, though their protection has shifted as the virus has evolved. The updated 2024-2025 vaccine reduces hospitalizations in adults 65 and older by roughly 45%, according to CDC data from the current season. That number is lower than the 90%-plus efficacy seen in the original clinical trials, but the vaccines continue to provide meaningful protection against the outcomes that matter most: hospital stays, intensive care, and death.
How Well the Current Vaccine Works
The 2024-2025 COVID-19 vaccine, reformulated to target recent Omicron subvariants, was studied across two CDC surveillance networks from September 2024 through January 2025. Among immunocompetent adults 65 and older, effectiveness against COVID-related hospitalization was 45% to 46% within the first four months after vaccination. For immunocompromised adults in the same age group, that figure was slightly lower at 40%.
These numbers reflect real-world conditions where most people have some degree of prior immunity from past infections, making it harder for any vaccine to show dramatic additional benefit on top of what the immune system already knows. Hospitalization rates during this period were also relatively low compared to previous years, which meant there wasn’t enough data to calculate protection against death or ICU admission specifically.
Protection against getting infected at all is considerably lower than protection against hospitalization. This has been true since the Omicron era began. The virus mutates quickly in the regions that your antibodies target, so a vaccinated person can still catch COVID. But the deeper layers of immune defense, the ones that prevent the virus from overwhelming your lungs and organs, hold up much better and for longer.
Why Hybrid Immunity Matters
If you’ve both been vaccinated and had a prior COVID infection, you carry what researchers call hybrid immunity, and it’s substantially stronger than either alone. A large systematic review published in The Lancet Infectious Diseases found that hybrid immunity provided 97.4% protection against hospitalization or severe disease at 12 months after a primary vaccine series, and 95.3% at six months after a booster dose.
Protection against reinfection, however, faded much faster. Hybrid immunity dropped to about 42% effectiveness against catching the virus again at the 12-month mark. This pattern reinforces a consistent theme: your immune system gets very good at recognizing COVID and mounting a fast response that keeps you out of the hospital, even as the virus evolves enough to cause mild breakthrough infections.
Protection for Children
COVID vaccines have been particularly effective at preventing a rare but dangerous complication in children called Multisystem Inflammatory Syndrome, or MIS-C, a condition where multiple organs become inflamed weeks after infection. CDC data from 24 pediatric hospitals found that two doses of vaccine were 91% effective at preventing MIS-C in children aged 12 to 18. Every single one of the 38 MIS-C patients who needed life support during the study period was unvaccinated.
Vaccine effectiveness against COVID-related hospitalization in this age group was similarly high at 93%, though these figures come from the Delta and early Omicron period. Effectiveness against current variants in children is expected to follow the same general pattern seen in adults: strong protection against severe disease, more modest protection against infection.
Protection for Pregnant People and Newborns
Babies under six months are too young to be vaccinated, but they can receive protection through maternal antibodies. CDC data showed that completing a two-dose vaccine series during pregnancy reduced COVID-related hospitalization in infants under six months by 61%. Timing made a significant difference: vaccination later in pregnancy (after 20 weeks) was 80% effective at protecting the infant, while earlier vaccination provided a less reliable benefit. This makes sense because antibody transfer across the placenta is most efficient in the third trimester.
Reducing the Risk of Long COVID
Beyond preventing acute illness, vaccination also lowers the chance of developing persistent symptoms after infection. A literature review by the European Centre for Disease Prevention and Control found that full vaccination before a first infection reduced the risk of long COVID by approximately 27%. That’s a modest but real reduction for a condition that can cause fatigue, brain fog, and exercise intolerance lasting months or longer. Combined with the protection against severe acute illness, this adds another layer to the overall benefit of vaccination.
Myocarditis Risk in Context
Heart inflammation (myocarditis) has been one of the most discussed side effects of mRNA COVID vaccines, particularly in younger males. A major study in children and young people put the risk in direct comparison: COVID infection caused an estimated 2.24 extra cases of myocarditis or pericarditis per 100,000, while vaccination caused 0.85 extra cases per 100,000. In other words, the infection itself was roughly 2.6 times more likely to trigger heart inflammation than the vaccine.
The risk profile also differed in duration. Vaccine-associated myocarditis risk was limited to the first four weeks after the shot, with no increased risk beyond that window. COVID-related myocarditis risk was highest in the first week after diagnosis, with a 3.5-fold increase compared to baseline. Most cases of vaccine-related myocarditis in young people have been mild and self-resolving, though they warrant monitoring.
Why Effectiveness Numbers Change Over Time
The shift from 95% efficacy in 2020 clinical trials to 45% real-world effectiveness in 2024 reflects several overlapping factors. The virus has mutated substantially, with current variants bearing little resemblance to the original strain the first vaccines targeted. Population-wide immunity from prior infections means the baseline comparison has changed: “unvaccinated” no longer means “immunologically naive.” And vaccine protection naturally wanes over months as antibody levels decline, even though immune memory cells persist and continue to offer defense against severe outcomes.
Updated vaccines are reformulated annually, similar to the flu shot, to better match circulating strains. The goal has shifted from preventing all infections to maintaining a durable shield against hospitalization and death, which the current vaccines continue to do at clinically meaningful levels.