Fact check: How do side effects of the Moderna Covid vaccine compare to the Johnson & Johnson vaccine?

1. Why the headlines differ: efficacy versus safety drama

The contrast between headlines about efficacy and headlines about safety springs from different emphases in the source studies: one review highlighted Moderna’s 94.1% and Pfizer’s 95.0% efficacy versus symptomatic COVID-19, while Johnson & Johnson’s trial-level efficacy was reported at 66.0% for moderate-to-severe disease ^[1]. At the same time, broader comparative work across multiple vaccines emphasized relative and absolute risk reductions and concluded that none of the vaccines showed a statistically significant rise in serious adverse events versus placebo ^[2]. These dual emphases produce seemingly conflicting narratives: higher efficacy numbers for mRNA vaccines but overall reassuring safety signals across platforms ^{[1] [2]}.

2. What the reviews say about common side effects: reactogenicity is expected

Across the systematic overviews, the consistent finding is that local and systemic reactogenic effects — injection-site pain, fatigue, headache, myalgia, fever — were common but generally short-lived after vaccination with both mRNA and viral-vector vaccines. The meta-analysis covering multiple vaccine platforms concluded vaccines were immunogenic and safe with adverse events being rare overall, implying that the common side effects were not classified as serious adverse events by trial definitions ^[3]. This pattern suggests that every authorized vaccine carried predictable, usually transient side effects, with no major safety outliers in the trial data pooled by these reviews ^{[2] [3]}.

3. Serious adverse events: rare and not clearly different across vaccines

All three analyses reached the same core conclusion that serious adverse events were uncommon and did not differ significantly from placebo groups in the compiled clinical-trial data. The comparative study that explicitly assessed five major vaccines found no significant increase in serious adverse events for any vaccine compared to placebo, a key point when weighing vaccine risk profiles ^[2]. The systematic review and meta-analysis reinforced that high-quality trial evidence showed vaccines were safe and immunogenic, while acknowledging that absolute rates of rare events are difficult to estimate precisely from trials alone ^[3].

4. Why efficacy numbers matter but don’t tell the whole safety story

Efficacy percentages reported in trials — for example 95% for Pfizer, 94.1% for Moderna, and 66% for Johnson & Johnson — measure prevention of symptomatic or moderate-to-severe disease under trial conditions, not rates of adverse events ^[1]. While higher efficacy can influence public perception of value, the systematic comparisons stress that efficacy and safety are distinct outcomes: a vaccine can be more efficacious yet carry a similar safety profile to a less efficacious one. The reviews emphasize that trial endpoints, population risk, and circulating variants at the time of study affect efficacy estimates, complicating direct safety-to-efficacy tradeoffs ^{[1] [3]}.

5. Limitations that matter: trials, timing, and rare events

All three sources note methodological caveats that affect interpretation; trial populations, follow-up durations, and differing definitions of adverse events limit direct comparisons. Meta-analyses and comparative reviews can obscure rare but important events because randomized trials are generally underpowered to detect extremely rare safety signals. The analyses implicitly warn that post-authorization surveillance is essential to capture rare adverse events and to assess safety across broader, more diverse populations than trial cohorts ^{[2] [3]}. These limitations underscore why regulatory decisions rely on both trial data and ongoing monitoring.

6. Different narratives, possible agendas, and the role of framing

The reviewed studies come from perspectives that emphasize different metrics: one highlights efficacy rankings, another stresses population-level benefit and risk balance, and a third focuses on immunogenicity and pooled safety evidence ^{[1] [2] [3]}. These differing priorities can reflect institutional aims—public health guidance prioritizes population impact, whereas clinical reviews may foreground immunogenicity. Readers should recognize that framing (efficacy first vs. safety-first narratives) shapes impressions about which vaccine “looks better,” despite consistent trial findings that serious harms were rare across platforms.

7. Bottom line for a concerned individual: expected side effects and rare risks

The consolidated message across these analyses is that Moderna and Pfizer produce similar, commonly transient reactogenic side effects and high efficacy, while Johnson & Johnson shows lower efficacy in trials but comparable rates of serious adverse events in the studied populations ^{[1] [2] [3]}. For personal decision-making, the reviews suggest focusing on expected short-term side effects, overall protective benefits, and the need for post-marketing data on rare events, rather than assuming large safety differences between platforms based solely on efficacy headlines ^{[2] [3]}.

8. What’s missing and what to watch next

The analyses collectively indicate that more granular, long-term, and post-authorization surveillance data are needed to refine estimates of very rare adverse events and to assess performance across variants and subpopulations. Going forward, readers should watch for real-world safety surveillance reports and updated meta-analyses that stratify by age, sex, comorbidity, and variant context, which will better illuminate any small differences in rare safety outcomes that trials could not detect ^{[2] [3]}. These ongoing data will complete the picture sketched by the trial-focused reviews cited here.