An interesting preprint by Petra and others in medRxiv looks at the evidence for lower viral load of individuals who have been vaccinated with the Pfizer vaccine.
One of the key questions regarding COVID-19 vaccines is whether they reduce viral shedding.
To date Israel vaccinated substantial parts of the adult population which enabled extracting real world signals.
The vaccination rollout started on December 20th 2020, utilised mainly the Pfizer vaccine and focused on individuals who were 60 years or older.
By now more than 75% of the individuals of this age group that have been at least 14 days after the first dose, compared to 25% of the individuals between 40-60 years old.
This study traced the CT value distribution of 16,297 positive PCR tests in the laboratory between December 1s to January 31st that came from these two age groups.
As the laboratory did not have access to the vaccine status of each test, the hypothesis was if the vaccine reduced viral load, we should see a difference in the CT values between the two age groups in late January but not before.
Consistent with this hypothesis, until January 15th they did not find any statistically significant differences in the average CT values between the groups.
In stark contrast, the results in the last two weeks of January showed a significant weakening in the average CT value of 60 plus individuals to the 40-60 age group.
To further corroborate these results, they also nested linear models to explain the CT values of the positive test.
This analysis favoured a model that included an interaction between age in the late January time period, consistent with the effect of vaccination.
They then used demographic data and the daily vaccination rates to estimate the effect of vaccination on the viral load reduction.
This study suggested that vaccination reduced the viral load by 1.6 x 20 times in individuals who are positive for SARS-CoV-2. Taking together, the findings indicate vaccination is not only important for the individual’s protection but also reduces transmission.
LGP, the leading London doctors’ clinic in Harley Street commends the Government on its vaccination programme and encourages all those eligible to be vaccinated.
Dr Paul Ettlinger
BM, DRCOG, FRCGP, FRIPH, DOccMed