The Johnson & Johnson (Janssen) Ad26.COV2.S vaccine was the only single-dose COVID-19 vaccine authorized for use in the United States, offering a logistically simpler alternative to the two-dose mRNA regimens from Pfizer and Moderna. It used a modified adenovirus to deliver instructions for making the SARS-CoV-2 spike protein, training the immune system to recognize the real virus. The vaccine’s single-shot convenience came with trade-offs in overall efficacy, and its story reveals a great deal about how COVID vaccination strategy evolved and where it may be headed.
How the Adenoviral Vector Platform Works
The J&J vaccine relied on a human adenovirus, specifically adenovirus type 26, as a delivery vehicle. Adenoviruses are common cold viruses, and the one used in the vaccine was engineered so it could not replicate inside your body. Its job was to carry genetic instructions for the SARS-CoV-2 spike protein into your cells. Once inside, your cells read those instructions, produced spike proteins on their surfaces, and your immune system mounted a response against them. This is different from mRNA vaccines, which deliver the genetic instructions directly as messenger RNA wrapped in lipid nanoparticles rather than inside a virus shell.
The adenovirus itself also triggers some innate immune activation. Adenoviruses interact with multiple immune sensing systems in the body, including receptors on cell surfaces and inside cells, which contributes to the strong initial immune reaction many people experienced after injection.1PubMed Central. Adenovirus vector induced innate immune responses: impact upon efficacy and toxicity in gene therapy and vaccine applications That built-in immune-stimulating property is part of why adenoviral vectors have been used in vaccine research for decades. It also helps explain why side effects like fever, fatigue, and muscle aches were common in the first day or two after the shot.
What the Clinical Trials Found
The pivotal trial for Ad26.COV2.S enrolled tens of thousands of participants across multiple countries. Early results showed the vaccine was about 67% effective at preventing moderate to severe COVID-19 at least 14 days after vaccination, with higher protection against the most serious outcomes. For severe to critical illness specifically, efficacy reached roughly 77% after 14 days and about 85% after 28 days.2PubMed Central. Safety and Efficacy of Single-Dose Ad26.COV2.S Vaccine against Covid-19 Those numbers were reassuring: even if the vaccine did not prevent every infection, it was quite good at keeping people out of the hospital.
A final analysis of the same large trial, which captured more data including the period when the Delta variant was spreading, confirmed the overall pattern but with somewhat lower numbers for moderate to severe disease. Efficacy against moderate to severe COVID-19 was about 56% at 14 days and roughly 53% at 28 days. Protection against severe to critical disease held up better at about 75%, and the vaccine was roughly 83% effective against COVID-19-related death. That protection lasted six months or longer.3PubMed Central. Final Analysis of Efficacy and Safety of Single-Dose Ad26.COV2.S The gap between the initial and final analyses reflected the reality of new variants eroding vaccine effectiveness over time, a pattern that affected all COVID vaccines.
Real-World Effectiveness
Clinical trials are controlled environments, so real-world data filled in a more practical picture. A large U.S. study comparing nearly 400,000 vaccinated individuals to over 1.5 million matched unvaccinated people found that the single-dose J&J vaccine was about 79% effective against COVID-19 infection and 81% effective against COVID-related hospitalization.4medRxiv. Effectiveness of the Single-Dose Ad26.COV2.S COVID Vaccine Those real-world numbers were actually higher than the trial results, likely because the study period captured a time before the Delta wave had fully taken hold.
The practical appeal of a single-dose vaccine was enormous. For people who had difficulty scheduling two appointments, lived far from vaccination sites, or were transient, one shot and done was a genuine advantage. Research on vaccine hesitancy found that having the J&J option available may have helped some people get vaccinated who otherwise would not have, since it removed the barrier of returning for a second dose.5Journal of Applied Research in Memory and Cognition. Having options can lower vaccine hesitancy, finds research
Rare but Serious Safety Signals
The J&J vaccine’s safety profile included the usual side effects you would expect from any vaccine: sore arm, fatigue, headache, fever, and muscle pain, all of which resolved within a day or two for most people. The more consequential story involved two rare but serious conditions that emerged after authorization.
The first was vaccine-induced immune thrombotic thrombocytopenia, or VITT, a condition where the body develops unusual blood clots paired with a dangerous drop in platelet count. Patients with VITT typically developed symptoms between 5 and 24 days after vaccination. By mid-2021, after nearly two billion COVID vaccine doses had been given worldwide, about 467 VITT cases had been reported globally across all adenoviral vector vaccines, representing an extraordinarily small fraction of recipients.6PubMed Central. To clot or not to clot? Ad is the question-Insights on mechanisms related to vaccine-induced thrombotic thrombocytopenia The condition was more commonly reported among younger women, and it appeared to involve antibodies against a protein called platelet factor 4, triggering a clotting cascade. The exact mechanism by which the adenoviral vector provoked this response was never fully worked out, though researchers proposed several theories involving the virus vector’s interaction with platelets and endothelial cells.
The second rare condition was Guillain-Barré syndrome, a neurological disorder where the immune system attacks peripheral nerves, causing weakness and sometimes paralysis. Surveillance data from the U.S. Vaccine Safety Datalink showed that the rate of confirmed GBS cases in the first three weeks after the J&J vaccine was significantly higher than the background rate in the general population.7JAMA Network Open. Incidence of Guillain-Barré Syndrome After COVID-19 Vaccination in the Vaccine Safety Datalink A nationwide study estimated about 5.7 attributable GBS cases per million J&J doses administered.8PubMed Central. Risk of Guillain-Barré Syndrome Following COVID-19 Vaccines: A Nationwide Self-Controlled Case Series Study That risk was elevated compared to background but still very low in absolute terms. GBS was not linked to the mRNA vaccines at comparable rates, which made it an adenoviral-vector-specific concern.
These safety signals, particularly VITT, were the primary reason U.S. health authorities eventually moved away from recommending the J&J vaccine when mRNA alternatives were readily available.
When One Dose Worked Especially Well
One of the most interesting findings from the pandemic was that a single vaccine dose performed dramatically better in people who had already been infected with SARS-CoV-2. If you had recovered from COVID and then got one shot, your immune response looked as good as, or often better than, what someone without prior infection achieved after two doses.
Research on healthcare workers showed that after a single dose, people with prior infection had enhanced T cell immunity, stronger memory B cell responses against the spike protein, and neutralizing antibodies that worked against multiple variants, including ones that partially evaded vaccine-only immunity.9PubMed Central. Prior SARS-CoV-2 infection rescues B and T cell responses to variants after first vaccine dose By contrast, people receiving one dose without prior infection showed weaker protection against variants. A separate study found that antibody levels after a single mRNA dose in previously infected people were comparable to those after two doses in people who had never been infected.10PubMed Central. Prior COVID-19 Infection and Antibody Response to Single Versus Double Dose mRNA SARS-CoV-2 Vaccination
This hybrid immunity, the combination of natural infection and vaccination, turned out to be one of the most durable forms of protection observed throughout the pandemic. A systematic review found that among people with prior infection, a single mRNA vaccine dose provided effectiveness ranging from 32–89% against infection, 57–97% against symptomatic infection, and 80–94% against hospitalization, depending on the variant and time period studied.11Communications Medicine. Effectiveness of a single COVID-19 mRNA vaccine dose in individuals with prior SARS-CoV-2 infection: a systematic review By the time Omicron subvariants arrived, prior infection combined with a single dose still offered meaningful added protection, though with wider ranges reflecting the virus’s increased ability to dodge immunity.
One Dose Versus Two for Long-Term Protection
For people without prior infection, the single-dose approach carried a real disadvantage compared to a full two-dose mRNA course. A study of over 200,000 individuals who had previously been infected found that even among this already-primed group, those who received only one vaccine dose had about a 69% higher risk of breakthrough infection compared to those who received two doses.12PubMed Central. One dose versus two doses of COVID-19 vaccine for the prevention of breakthrough infections among people previously infected with SARS-Cov-2 Unvaccinated individuals had about three times the breakthrough infection risk compared to the two-dose group. The message was clear: two doses provided a measurable advantage in preventing infections, even among people whose immune systems had already encountered the virus.
This finding contributed to the recommendation that people who initially received the J&J vaccine should get a booster dose, ideally with an mRNA vaccine. The “mix and match” approach, getting J&J followed by a Moderna or Pfizer booster, turned out to produce strong immune responses. For certain populations with weakened immune systems, such as people with blood cancers, research specifically suggested that J&J recipients should receive additional doses or heterologous mRNA boosters to achieve adequate protection.13PubMed Central. COVID-19 Vaccine Responses in Patients With Plasma Cell Dyscrasias After Complete Vaccination
Where Things Stand Now
The J&J vaccine is no longer authorized for use in the United States. The shift happened gradually: first the CDC expressed a preference for mRNA vaccines over J&J in late 2021, then in May 2023 the vaccine’s emergency use authorization was officially pulled. The decision reflected a combination of the rare safety signals and the availability of mRNA vaccines that offered stronger and more sustained protection.
The current U.S. COVID vaccination landscape has moved entirely to updated formulations. In September 2023, the FDA authorized monovalent vaccines with an XBB.1.5 component from Moderna and Pfizer-BioNTech, along with an updated Novavax protein-based vaccine. Earlier formulations, including the original bivalent mRNA vaccines and the original Novavax vaccine, were pulled from use.14CDC MMWR. Use of Updated COVID-19 Vaccines 2023–2024 Formula for Persons Aged ≥6 Months: Recommendations of the Advisory Committee on Immunization Practices — United States, September 2023 The ACIP recommended the updated vaccine for everyone aged six months and older. COVID vaccination has settled into a pattern more like the annual flu shot: periodic updates to match circulating strains, with most people needing a single updated dose each season.
Safety During Pregnancy
One question that caused considerable anxiety throughout the pandemic was whether COVID vaccines were safe for pregnant people. A systematic review and meta-analysis examining all currently administered COVID-19 vaccines, including the adenoviral vector platforms, found no safety concerns during pregnancy.15PubMed Central. Safety of COVID-19 vaccines during pregnancy: A systematic review and meta-analysis That included no increased risk of miscarriage, preterm birth, stillbirth, or adverse neonatal outcomes compared to unvaccinated pregnant people. Given that COVID-19 infection itself poses elevated risks during pregnancy, including higher rates of preterm delivery and ICU admission, vaccination during pregnancy was broadly recommended by obstetric and public health organizations.
Intranasal Vaccines and the Future of Single-Dose Strategies
The idea of a single-dose COVID vaccine has not disappeared with J&J. Several research groups are working on intranasal vaccines, which would be sprayed into the nose rather than injected. The theoretical advantage is compelling: by delivering the vaccine directly to the respiratory mucosa where the virus first enters the body, intranasal vaccines could trigger local immune responses, including secretory IgA antibodies in the nasal passages, that might prevent infection and viral shedding more effectively than an injected vaccine.16PubMed Central. COVID-19 intranasal vaccines: current progress, advantages, prospects, and challenges
Early real-world results have been mixed. A study of a single-dose intranasal spray vaccine in healthcare workers during an Omicron wave found modest protection against symptomatic infection overall, with adjusted effectiveness of about 19%. However, it performed considerably better against more severe symptoms, with about 59% effectiveness against grade 3 or higher illness. Among people who had never received any prior COVID vaccination, the intranasal vaccine showed much higher effectiveness against symptomatic infection, around 87%.17The Lancet Regional Health – Western Pacific. Short-term effectiveness of single-dose intranasal spray COVID-19 vaccine against symptomatic SARS-CoV-2 Omicron infection in healthcare workers: a prospective cohort study The pattern suggests that intranasal vaccines may face the same challenge as injected ones: layering additional vaccine-induced immunity on top of extensive prior exposure to the virus and prior vaccination yields smaller marginal gains.
Researchers are also exploring other single-dose platforms. One animal study demonstrated that an adeno-associated virus (AAV)-based COVID vaccine, different from the adenovirus used by J&J, produced neutralizing antibodies that remained stable for a full year in non-human primates, alongside functional memory T cell responses.18PubMed Central. An AAV-based, room-temperature-stable, single-dose COVID-19 vaccine provides durable immunogenicity and protection in non-human primates That platform has an added logistical benefit: room-temperature stability, which would eliminate the cold chain requirements that made distributing mRNA vaccines so challenging in resource-limited settings.
Stretching Supply Through Fractional Dosing
A related but distinct concept is fractional dosing, where a smaller-than-standard dose of vaccine is given to stretch limited supplies further. This was explored during the pandemic when vaccine scarcity was a global crisis. A modeling study focused on India found that fractional booster doses, as small as one-eighth of a standard dose, could be the most cost-effective strategy under various transmission scenarios, generating billions of dollars in net health benefit compared to full-dose approaches. The savings came from vaccinating far more people with the same supply, even at slightly reduced per-person effectiveness.19PubMed Central. Cost effectiveness of fractional doses of COVID-19 vaccine boosters in India Fractional dosing has precedent with other vaccines, including yellow fever, where a fifth of the standard dose was used during outbreaks in Africa and Brazil with WHO endorsement.
The appeal of single-dose and fractional-dose strategies goes beyond individual convenience. During a pandemic, the speed at which a population reaches meaningful levels of immunity matters enormously. A single-dose regimen that gives moderate protection to twice as many people in a given timeframe can prevent more total disease and death than a two-dose regimen that gives stronger protection to half as many. That calculus shifts once supply is plentiful, but during scarcity it drove real policy decisions about how to allocate limited vaccine stocks.
What the J&J Experience Taught Us
The J&J vaccine occupied an unusual niche in pandemic history. It was authorized later than the mRNA vaccines, accumulated rare safety signals that the mRNA vaccines did not share, and ultimately lost its authorization entirely. But its contribution was not trivial. Millions of people received it worldwide, and it provided meaningful protection against severe disease and death during a period when those outcomes were common. For many, it was the only vaccine they were willing or able to get.
The broader lesson is about vaccine platform diversity. Having multiple vaccine technologies available simultaneously, adenoviral vectors, mRNA, and protein subunit, allowed public health systems to adapt when problems emerged with one platform. It also revealed that the concept of “single dose” is really about the interaction between the vaccine and the recipient’s immune history. As virtually the entire global population has now been exposed to SARS-CoV-2 through infection, vaccination, or both, the distinction between single-dose and multi-dose regimens matters less than it did in early 2021. A single updated dose of an mRNA vaccine now serves as a booster for a population with extensive prior immune priming, functionally accomplishing what the J&J vaccine originally set out to do: one shot, meaningful protection, move on.