Follow-Up Research Provides Key Insights into COVID-19 Vaccine Efficacy

Large-scale follow-up studies conducted on those vaccinated by the Pfizer-BioNTech COVID-19 vaccine shed new light on how long antibodies last, effects on types of patients, and effectiveness against the Delta variant.

Israel, which has been at the forefront of vaccinating its citizens, has also carried out extensive follow-up studies on the effects and effectiveness of the Pfizer-BioNTech COVID-19 vaccine.

In an analysis of more than 26,000 blood samples, researchers at Tel Aviv University (Tel Aviv, Israel) and Shamir Medical Center (Be'er Ya'akov, Israel) found that COVID-19 antibody levels vary according to age and gender. Their study revealed that symptomatic females infected with COVID-19 had the strongest immune reaction after the age of 50 years while symptomatic infected males showed the strongest immune reaction at the age of around 35 years. The scientists have attributed these differences to hormones. The researchers also found that vaccinated people had an immune response that was four times stronger as compared to COVID-19 patients who had recovered.

In another study of 17 hospitals, researchers at Samson Assuta Ashdod Hospital (Ashdod, Israel) found that older individuals who had several underlying medical conditions and immunosuppression were more prone to SARS-CoV-2 infection and severe COVID-19 despite being fully vaccinated.

A team led by Sheba Medical Center (Ramat Gan, Israel) also undertook a study to explore whether mRNA vaccines such as Pfizer’s negatively impacted fertility. Their study revealed that fully vaccinated couples exhibited no differences in sperm quality, semen volume, ovarian stimulation or proportion of high-quality embryos in comparison to their IVF cycles before vaccination. Similarly, a study by the Hadassah-Hebrew University Medical Center (Jerusalem, Israel) found that the COVID-19 vaccine did not have any effect on sperm parameters.

Another study conducted by Sheba Medical Center in collaboration with the Pasteur Institute (Paris, France) and Sorbonne Université (Paris, France) found that among all vaccinated members living in the same house, the risk of contracting COVID-19 for any household member declined from 57% to 4%. Even a single household member inoculated with the Pfizer vaccine significantly reduced the risk to unvaccinated household members. This confirmed findings from an earlier study that had indicated fully vaccinated individuals appeared to enjoy protection from COVID-19 even after being exposed to family members who were infected.

The effectiveness of the Pfizer COVID-19 vaccine in preventing infections and symptomatic illness declined to 64% in early July from a high of 95% reported in May by the Israeli Health Ministry. This can be attributed to the entry of the Delta variant into the country. The same ratio fell further to about 40% as per a report released July 22, most likely due to the effects of the vaccinations administered in January and February wearing off over time. The early July report had stated that the vaccine was 93% effective in preventing serious illness from COVID-19, while this number the late July report reported a drop in effectiveness to 91%. Additionally, the ministry also found that there was a one-third lower chance of healthy vaccinated individuals developing severe COVID-19 as compared to healthy unvaccinated individuals.

These findings were further confirmed by an analysis of medical records of tens of thousands of members of Israeli HMO Leumit Health Fund that indicated individuals who had been vaccinated before late February faced double the risk of a breakthrough SARS-CoV-2 infection as compared to people vaccinated since late February. However, it could not be confirmed whether this was due to the majority of the early vaccinated being elderly people and/or those with chronic health issues, or on account of waning effects of the Pfizer vaccine.

A panel of Israeli researchers reported to the Ministry of Health that rare cases of myocarditis, a heart muscle inflammation, were seen in men below the age of 25 years who had taken the second dose of the Pfizer-BioNTech COVID-19 vaccine. However, the majority of the cases were mild and resolved within a few weeks. In late July, the Health Ministry reported that 80% of vaccinated individuals who experienced breakthrough COVID-19 infection did not infect people with whom they came in contact with at public places.

Studies carried out during the start of Israel’s vaccination drive found that immunity from the Pfizer-BioNTech COVID-19 vaccine was the strongest 14 days after the first of two doses were administered. The findings also showed that individuals who became infected after taking the first dose demonstrated a significantly lower risk of transmitting the virus to other people. Additionally, the first vaccine dose was found to boost immunity even in individuals who had recovered from COVID-19.

In another study, employees of Sheba Medical Center who had taken both doses of the Pfizer vaccine were found to have higher levels of antibodies against SARS-CoV-2 as compared to people who had recovered from COVID-19. Meanwhile, a small study at Tel Aviv Sourasky Medical Center (Tel Aviv, Israel) found antibodies being produced in the breastmilk of nursing mothers who were fully vaccinated, thus potentially protecting their babies. In other findings, a study conducted in February showed that individuals who had received Pfizer-BioNTech COVID-19 vaccine were significantly less likely to “shed” or transmit virus particles present in their nose or throat.

Another important Sheba Medical Center study showed that among 11,453 fully vaccinated healthcare workers in the medical center, 39 breakthrough cases of COVID-19 were detected through RT-PCR testing of 1,497 of the vaccinated workers between January 20 and April 28. Out of these 39 breakthrough cases of COVID-19, the suspected source of infection was an unvaccinated person in 37 cases. None of the infected workers needed hospitalization, although 19% reported having long COVID-19 symptoms at six weeks after their diagnosis, including prolonged loss of smell, persistent cough, fatigue, weakness, dyspnea, or myalgia.

The above study results appeared in the online publication ISRAEL21c, which featured a synopsis of the scientific findings that were reported during January-July, 2021.

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Tel Aviv University