Prevnar Clinical Trial Adverse Events & Data

What adverse events were tracked in the Prevnar clinical trials? This page documents the pre-licensure clinical trial data for every pneumococcal conjugate vaccine approved in the United States.

Clinical trial data for the four pneumococcal conjugate vaccines licensed for the US childhood schedule. The original product (Prevnar 7) was licensed against an unlicensed experimental meningococcal conjugate vaccine — a fact even FDA and CDC scientists conceded undermines safety inference. Each successor was licensed against its predecessor, with SAE rates escalating from 7.2% (Prevnar 7) to 9.6% (Vaxneuvance) — each accepted because it was similar to the previous generation.

Prevnar 7 (Pfizer/Wyeth) — PCV-7

Source: FDA licensure review; published commentary by FDA and CDC scientists.

The Experimental Vaccine "Control"

Prevnar 7's trial control was literally described as "an investigational meningococcal group C conjugate vaccine" — an unlicensed experimental product that had never itself been tested against a placebo.

Aaron Siri: "Literally, it was 'an investigational meningococcal group C conjugate vaccine,' meaning an unlicensed experimental vaccine. Seriously, I couldn't have even dreamed of making this up."

FDA/CDC Scientist Concession

FDA and CDC scientists who reviewed the trial acknowledged in published commentary: "the control group in [Prevnar 7's] main study received another experimental vaccine, rather than a placebo. If both vaccines provoked similar adverse effects, little or no difference between the 2 groups might have been evident."

Despite this explicit acknowledgment that the trial design could not detect adverse effects shared between the two products, the vaccine was licensed.

Prevnar 13 (Pfizer) — PCV-13

Source: Pfizer's FDA-approved package insert.

Pfizer's Defense of the Elevated SAE Rate

Pfizer's own FDA-approved package insert acknowledged the 6-month monitoring window "may have resulted in serious adverse events being reported in a higher percentage of subjects than for other vaccines" — offered as an explanation for the elevated rate.

Siri: "Not an excuse — an indictment." The 6-month window that revealed the 8.2% SAE rate was used to excuse the finding rather than investigate it. The FDA accepted this reasoning and licensed Prevnar 13.

Vaxneuvance (Merck) — PCV-15

Source: FDA-approved package insert; clinical trial submission.

"No Notable Patterns" Justification

Despite the elevated SAE rate, the FDA deemed Vaxneuvance safe because there were "no notable patterns or numerical imbalances between vaccination groups." The 9.6% vs. 8.9% difference was treated as within acceptable bounds — benchmarked against a control with an 8.9% SAE rate, which was itself benchmarked against Prevnar 7 (7.2%), which was benchmarked against an experimental vaccine.

Prevnar 20 (Pfizer) — PCV-20

Source: FDA-approved package insert.

Split Reporting Obfuscation

By the time Prevnar 20 was licensed, the SAE rates from prior PCV trials had become high enough to attract scrutiny. The Prevnar 20 trial divided adverse events into two separate reporting categories — "serious adverse events" and "newly diagnosed chronic medical conditions" — rather than reporting them together as prior trials had done. This split reporting makes direct comparison to prior PCV SAE rates misleading.

Siri: "Deemed 'safe' because 'no notable patterns or imbalances between vaccine groups.'"

Summary Table

Escalating SAE Pattern

The PCV pyramid illustrates how SAE rates escalate through successive "as safe as" comparisons:

```

Prevnar 7 control: unlicensed experimental vaccine (SAE rate concealed)

↓

Prevnar 13: 8.2% SAE rate (vs. Prevnar 7's 7.2%)

↓

Vaxneuvance: 9.6% SAE rate (vs. Prevnar 13's 8.9%)

```

Each higher SAE rate is accepted because it doesn't significantly exceed the prior generation's rate — which was itself never validated against placebo.

Schedule

Disease Context

Streptococcus pneumoniae causes invasive bacterial disease (meningitis, sepsis, bacteremia) and pneumonia. Before Prevnar, pneumococcal disease caused thousands of cases of invasive disease in children annually. Many serotypes are not covered by current PCV products, and serotype replacement (rise of non-covered serotypes filling the niche) has been documented in post-licensure surveillance. Each successive PCV (7 → 13 → 15 → 20) has added serotypes to address replacement.

See Also

PCV Vaccines (Post-Licensure), Pre-Licensure Safety Testing, Combination Vaccines (Pre-Licensure), Childhood Vaccine Schedule, Pfizer, Merck

Frequently Asked Questions

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