Pfizer-BioNTech COVID-19 vaccine associated with lower viral load in puncture infections

Several vaccines have been rapidly developed to help control the COVID-19 pandemic, including the BNT162b2 vaccine (Pfizer / BioNTech). This vaccine against severe acute respiratory syndrome with coronavirus 2 (SARS-CoV-2) has been shown in both clinical trials and the real world to be very effective in preventing infections, symptoms, hospitalization, severe illness and death associated with coronavirus disease (COVID -19).

Also, previous studies have shown a reduction in sudden infections due to the BNT162b2 vaccine. Following national vaccination campaigns worldwide, there has been a significant reduction in new SARS-CoV-2 infections, leading to hopes for herd immunity and eradication of the disease. For example, almost no new cases of COVID-19 have been detected in Israel since their vaccination campaign from April to June 2021.

However, there is an increase in new SARS-CoV-2 infections in highly vaccinated countries, including Israel. This recent increase in the rate of infection occurs in parallel with the global spread of the SARS-CoV-2 Delta variant and the possible reduction in the immune response elicited by the vaccine, raising questions about the effectiveness of the vaccine (VE) against the Delta variant over time. Recent studies show that VE against infection begins to decline six months after vaccination. However, protection against severe complications associated with COVID-19 appears to be sustainable.

A team of researchers from various institutions in Israel performed a retrospective analysis of measurements of reverse transcription-quantitative polymerase chain reaction (RT-qPCR) measurements of three SARS-CoV-2 genes (E, N and RdRp) from laboratory-confirmed positive tests collected from patients at Maccabi Healthcare Services. This study focuses on infections in adults over the age of twenty between June 28 and September 9, 2021, when the Delta variant was the most famous strain in Israel.

This study is available as a short release in Natural medicine diary.

Study: Viral loads of breakthrough infections with Delta-variant SARS-CoV-2 after vaccination and booster with BNT162b2.  Image credit: NIAID


This study showed that the BNT162b2 vaccine was initially effective in reducing the viral load in puncture infections with a ten-fold variant of Delta, which is consistent with the initial efficacy against the other variants. However, VE against viral load decreases over time after vaccination, decreases significantly three months after vaccination, and almost disappears six months after vaccination.

In Israel, when the Delta variant appeared, a large proportion of the vaccinated population passed the two-month period after vaccination, so the mean effect of the BNT162b2 vaccine for the entire population on the viral load of the Delta variant was negligible, which explains the reports of no difference in cycle threshold (Ct) between vaccinated and unvaccinated individuals. The authors found that with the use of booster vaccination, it was possible to restore the reduction in viral load even against the Delta variant, which implies a restoration of vaccine-related mitigation of transmission.

These results together suggest that VE in reducing the viral load of breakthrough infections of the Delta variant is similar to its observed efficacy in previous non-Delta jumps. The efficacy of the vaccine is reduced from the time of the initial vaccination, but can be restored by a third booster inoculation.

There were several limitations to the study. First, although viral load is a typical indicator of infectivity, a positive PCR test does not always mean that a viable virus is present, and the relationship between infectivity and viral load has yet to be established. Second, the results of recent studies show that vaccinated individuals show a faster decline in viral load than unvaccinated individuals, which may have an effect of differentiation on Ct values ​​for samples taken long after the onset of symptoms.

Therefore, it is also possible that differences in Ct values ​​between vaccinated and unvaccinated individuals are not only a representation of the initial difference in viral load, but also a difference in the breakdown of viral load between the two groups. Third, the authors did not know the number of asymptomatic and symptomatic cases within the study, and the proportion of symptomatic cases in the unvaccinated may be much higher.


The duration of the renewed effect of the booster vaccine on the reduction of sudden infections and whether additional booster doses will be needed in the future against currently available variants or new emerging variants remains unknown.

In addition, VE against severe infections may decrease at different time scales and may be affected differently by booster vaccines. However, to date, there has been only a slight decrease in the prevention of severe COVID-19. However, the lower viral load observed in booster-vaccinated puncture infections with Delta indicates a reduction in infectivity, which means that if this is accompanied by other precautions such as wearing masks, this may help control the spread of the pandemic.

Levine-Tiefenbrun, M., Yelin, I., Alapi, H. et al. Viral loads of delta variant SARS-CoV-2 breakthrough infections after vaccination and booster with BNT162b2. Nat Med (2021).

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