Pfizer Vaccine Effectiveness Drops to 39% Against Delta Variant: Is a Booster Shot Necessary?

In China, Nanjing epidemic intensifies; abroad, Pfizer vaccine efficacy drops significantly!

On July 28, Pfizer announced its second-quarter 2021 financial results, reporting first-half total revenue of $33.5 billion, a 68% increase year-over-year. Of this, COVID-19 vaccine sales accounted for $7.8 billion, bringing cumulative first-half sales to $11.3 billion, with full-year revenue projected to reach approximately $33.5 billion. Additionally, during the conference call, clinical progress was presented regarding the performance of the mRNA COVID-19 vaccine co-developed by Pfizer and BioNTech as a booster dose, demonstrating that a third dose increased neutralizing antibody titers by 5- to 8-fold.

However, a report recently released by the Israeli Ministry of Health has drawn significant attention. Samples collected between June 20 and July 17 show that the protective efficacy of two doses of the Pfizer (BioNTech) COVID-19 vaccine against the Delta variant is only 39%; this marks a substantial decline from the previously reported figures of 94.3% (between May 2 and June 5) and 64% (between June 6 and July 3).

01 Why does this happen?

According to the report, the 39% protective efficacy was calculated by aggregating multiple age groups. Furthermore, based on the reported vaccination timelines, the efficacy rate for cohorts vaccinated in January was 16%; for February, 44%; for March, 67%; and for April, 75%. It is worth noting that Israel prioritized the elderly and high-risk populations during its vaccination campaign. As the elderly—prioritized in January—typically mount a weaker antibody response compared to adults, there is a substantial likelihood that vaccine efficacy will wane rapidly over time.

Industry analysts point out that the suboptimal efficacy data for the Pfizer vaccine is largely attributable to age. Strictly speaking, the reported 39% efficacy rate should be stratified by age group to determine the specific efficacy for each cohort. However, this dataset lacks further granular analysis and merely represents aggregate follow-up data.

Meanwhile, researchers from the UK National Institute for Health Research, the University of Oxford, and other institutions published a paper in *The New England Journal of Medicine* stating that the Pfizer vaccine demonstrated an effectiveness of up to 88% in preventing symptomatic infection against the Delta variant, representing a twofold difference from the Israeli data.

Professor Cyril Cohen, a member of the Advisory Committee for COVID-19 Vaccine Clinical Trials at the Israeli Ministry of Health, explained the reasons during an interview with *The Jerusalem Post*, attributing them to factors such as differences in vaccination timing, the demographics of the vaccinated populations, and varying levels of viral testing.

Regarding differences in the vaccinated populations, it is also noted that in the UK, the Pfizer vaccine is primarily administered to individuals under 40, while the elderly predominantly receive the AstraZeneca vaccine. In Israel, however, the Pfizer vaccine is widely administered across different age groups. Individuals aged 60 and above generally exhibit weaker immune responses, and the waning of vaccine protection is more pronounced. Consequently, the immune barrier established by the vaccine is more likely to be breached by the Delta variant, and these individuals are more susceptible to developing symptoms following infection.

However, according to an Israeli report, the Pfizer vaccine remains effective in reducing the incidence of severe disease (88% effective against hospitalization and 91% effective against severe illness).

02 Will vaccines cause immune fatigue?

On the other hand, the Delta variant also indicates that prevention and control have become increasingly challenging. According to an article published in Nature, the viral load in individuals infected with the Delta variant can be up to 1,260 times higher than that in those infected with the original strain. Due to its rapid mutation and high transmissibility, it has become the dominant circulating strain globally. Vaccination remains the only effective method to prevent infection.

In light of the accelerated mutation of SARS-CoV-2, how long will the efficacy of the Pfizer vaccine or other vaccines last? Will waning immunity occur?

Another industry insider noted that it remains unclear whether vaccines will induce immune exhaustion against emerging viral variants. Furthermore, regarding antibody levels, both the likelihood and magnitude of antibody production are uncertain. Whether antibodies directly confer protection against the virus, whether a gold standard currently exists, and what it entails are also difficult to ascertain. In clinical trials, ethical constraints, coupled with researchers' evolving understanding of the virus and epidemic control measures, have indirectly influenced trial data, resulting in certain discrepancies between clinical trial outcomes and real-world data.

However, on April 1, Pfizer and BioNTech jointly released a six-month follow-up of over 40,000 clinical trial volunteers, demonstrating that the vaccine maintains an efficacy of 91.3% within six months, with a favorable safety profile.

Additionally, regarding previous inquiries in China about the duration of efficacy of domestically produced COVID-19 vaccines, relevant manufacturers similarly stated that antibody levels remain at a relatively high level more than six months after vaccination.

In the face of the surging Delta variant outbreak and declining vaccine efficacy, curbing the rapid mutation and spread of the strain has become an urgent priority. Previously, in response to the accelerated transmission of the Delta variant and the reduced vaccine effectiveness caused by viral mutations, Pfizer stated that administering a third vaccine dose could significantly boost antibody levels against SARS-CoV-2. Several countries have begun to consider a third "booster" dose, including China.

Currently, the COVID-19 situation in Nanjing remains severe, and the viral strain responsible for this local outbreak is the Delta variant (B.1.617.2). Given that the majority of the infected population has already been vaccinated, this inevitably raises new questions.

03 Should You Get the Booster Shot or Not?

A reporter from Yaozhi interviewed a vaccine expert, who stated that a decline in vaccine efficacy due to viral mutation is reasonable. Currently, all COVID-19 vaccines are facing reduced protection rates, which primarily depends on the specific nature of mutations in the viral target antigens targeted by these vaccines.

He emphasized that the primary goal of vaccine protection is to achieve herd immunity; once established, the transmission of infectious diseases becomes significantly hindered. Vaccine efficacy should be evaluated at multiple levels: population-level efficacy and individual-level efficacy. At the individual level, the ideal outcome of vaccination is the complete prevention of infection. In practice, vaccinated individuals may still experience breakthrough infections, which could be asymptomatic or, if symptomatic, result only in mild illness. Therefore, vaccination remains highly valuable. Although China has not yet issued official guidelines regarding booster doses, given the current severe epidemiological situation, administering booster shots is highly necessary.

According to previous disclosures by Sinovac, following two doses of the inactivated vaccine, administration of a third dose can rapidly increase antibody levels by 10-fold within one week, reaching 20-fold within half a month. Additionally, during a conference call on July 28, Pfizer stated that preliminary data from clinical trials indicate that six months after the second dose of the BNT162b2 vaccine, a third booster dose demonstrates a safety profile consistent with prior doses; against the wild-type virus, neutralizing antibody titers following the third dose increased by 5- to 8-fold compared to those after the second dose.

Hongtao Zhang, Research Associate Professor in the Department of Pathology and Laboratory Medicine at the Perelman School of Medicine, University of Pennsylvania, stated on his personal WeChat official account, "A third dose will definitely be necessary; that is not the question. The question is when to receive it."

Therefore, amid the continued global spread of the COVID-19 pandemic and the ongoing mutation of the Delta variant, a booster dose is essential regardless of whether the primary vaccination was Pfizer, Sinovac, or any other vaccine. Alternatively, as the novel coronavirus continues to mutate, humanity may need to achieve long-term coexistence with the virus; thus, it is entirely possible that booster shots, or even annual preventive vaccinations, will be required. These questions can only be resolved through scientific research, with clinical trials and data ultimately providing the answers.

However, although a third dose may be essential, the development and implementation of booster programs should comprehensively consider relevant factors such as local epidemiology, infection risk, and vaccine availability.

References:

1. Israel and the UK Respectively Release Pfizer Vaccine Efficacy Study Results: Twofold Difference (Hu Dingkun, Yu Ziyue/Science and Technology Daily)

2. Data on the Third Dose of the Sinovac Vaccine Has Been Released: What Are the Findings?

3. Pfizer Second Quarter Financial Report and Related Announcements

Executive Editor: Sanqi

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