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A new study investigates the waning protection of the Pfizer vaccine over 180 days. picture alliance/Getty Images
  • Researchers investigated whether COVID-19 immunity wanes after receiving a second Pfizer-BioNTech mRNA vaccine dose.
  • They found that the more time elapses following the second vaccine dose, the more likely individuals are to contract a SARS-CoV-2 infection.
  • While their results may warrant booster vaccines for immunocompromised individuals, they caution that further research is necessary because they did not use blood tests to monitor participants’ immune responses over time.

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Protection from COVID-19 comes from either contracting the SARS-CoV-2 virus and recovering or getting vaccinated.

Research has shown that those who have contracted SARS-CoV-2 have 85% protection against symptomatic disease 6 months after infection.

Meanwhile, as the authors of the recent study explain, “vaccination has been reported to be 50–95% effective at various time points.”

Over time, the immune system’s response to SARS-CoV-2 diminishes, meaning those who have recovered from the virus or received vaccination may be less protected as time progresses.

In a recent study, researchers from Israel and the United States conducted a study to investigate whether protection against infection waned over time following a second dose of the Pfizer-BioNTech mRNA vaccine.

They found that 90 days following a second Pfizer dose, individuals were at an increased risk of contracting SARS-CoV-2. The findings appear in the BMJ.

“Israel was one of the first countries to successfully roll out a vaccination campaign in the population, so we were also among the first to observe the waning effect of the vaccination as time elapsed since the initial two shots of the vaccine,” Dr. Ariel Israel, Ph.D., one of the study’s authors, told Medical News Today.

“Our study is an observational study, designed to evaluate whether protection provided by the Pfizer vaccine wanes with time by comparing the rate of positive [reverse transcription-polymerase chain reaction (RT-PCR) for SARS-CoV-2] in people vaccinated at different time intervals before the test.”

“We observed that after the excellent protection offered by the vaccine in the initial 3 months, there was a gradual increase in the infection rate,” Dr. Israel explained.

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The researchers gathered medical records from Leumit Health Services, a large healthcare provider in Israel that serves 700,000 people throughout the country.

For their analysis, they used health records from those aged 18 years and above who underwent a SARS-CoV-2 RT-PCR test between May 15, 2021, and September 17, 2021, after receiving two doses of the Pfizer vaccine.

The team performed tests at least 3 weeks following their second vaccination. They divided participants into three age groups to reflect different stages of the vaccine rollout:

  • 60 years and over
  • 40–59 years
  • 18–39 years

They also divided the time between the second vaccination and RT-PCR tests into 30-day intervals after the initial 90 days, with the last category denoted as 180 days or more since the second vaccination. Altogether, the researchers analyzed health records from 83,057 individuals.

Their analyses accounted for potential confounding risk factors for SARS-CoV-2 infection, such as age, sex, socioeconomic status, and existing health conditions.

Among those the researchers studied, 9.6%, or 7,973 individuals, had a positive test result. The average time between the second vaccine dose and an RT-PCR test was 164 days.

The team found that the more time elapsed since the second vaccine, the more likely people were to contract SARS-CoV-2.

While 1.3% of participants received a positive test result 21–89 days after their second vaccine, the same was true for 2.4% of people after 90–119 days, 4.6% after 120149 days, 10.3% after 150179 days, and 15.5% after 180 days.

These results translated into a 2.37-fold higher chance of contracting the virus after 90 days from the second vaccination and a 2.82-fold higher likelihood after 150 days or more.

They also found that two injections 21 days apart provided more protection than one and that immune system changes relating to age influenced the immune response to the vaccines.

Dr. Israel explained that the reasons for waning immune protection are “beyond the scope of their study.” However, he suggested some possible mechanisms:

“The most likely explanation is that antibodies, as well as cells of the immune system that produce antibodies or kill cells [with the infection], have a limited lifetime, so their numbers decrease gradually after the initial response triggered by the vaccine.”

“[Fewer] antibodies in the blood, and [fewer] cells able to kill the virus mean that the virus is more likely to evade the immune system in the first stages of infection, and this is probably why we observe an increased rate of positive PCR in individuals vaccinated earlier,” he added.

The researchers concluded that SARS-CoV-2 immunity following a second Pfizer vaccine wanes after the initial 90 days and that a third vaccine, or booster dose, may be warranted for immunocompromised individuals.

They also note several limitations to their study. Due to its observational study design, they say that they possibly did not account for all contributing factors, which may have skewed their results.

For example, they note they only included individuals who chose to request an RT-PCR test for SARS-CoV-2 and that some may have had different thresholds for requesting a test.

They also noted that individuals vaccinated earlier might have had different physical distancing habits than those vaccinated later, which could have significantly influenced their level of risk.

Moreover, the researchers note that RT-PCR tests were not followed up with blood tests, meaning they cannot be sure that immunity had indeed waned.

“In light of these results, public health authorities in Israel have recommended a booster shot for all age groups, and we have observed a subsequent dramatic reduction in COVID-19 incidence immediately following the rollout of the booster shot,” said Dr. Israel.

“It is too early to say for certain how long the protection would last following the booster shot. We are continuing to monitor the infection rate and will report our data in case we observe a reduction in the protection provided by the booster,” he concluded.

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