Protection against COVID-19 increases after fourth dose of vaccine but decreases rapidly

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The emergence and rapid spread of a new coronavirus, namely severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), has caused the ongoing coronavirus disease 2019 (COVID-19) pandemic. Many variants of SARS-CoV-2 have emerged around the world due to genomic mutations. They have been categorized into variants of concern (VOC) and variants of interest (VOI) due to their increased virulence and high transmission rates, as well as their ability to evade immune responses elicited by natural infection or vaccination. . The Omicron variant replaced the Delta variant as the dominant variant in circulation in many parts of the world.

Study: Protection by a fourth dose of BNT162b2 against Omicron in Israel. Image Credit: Gil Cohen Magen /shutterstock

context

Like many countries, Israel has seen an increase in the number of COVID-19 cases due to the rapid transmission of the Omicron variant. Two of the critical factors associated with the increased infection rate were the ability of this variant to evade immune protection and the passage of more than four months since most adults had received their third dose of the COVID-19 vaccine. . To reduce pressure on healthcare facilities, on January 2, 2022, Israeli authorities approved the administration of the fourth dose of the COVID-19 vaccine (BNT162b2), manufactured by Pfizer-BioNTech. All people over the age of sixty, those who are at high risk of contracting the disease or who have already passed more than four months since receiving the third dose, are given priority for the fourth dose of the vaccine.

The actual effectiveness of the four-dose COVID-19 vaccination strategy against confirmed infection and disease severity is not yet known. However, a new study published in The New England Journal of Medicine focused on the relative effectiveness of the fourth dose, compared to the three-dose vaccination regimen, against confirmed infections and serious illnesses in the elderly population of Israel.

Colorized scanning electron micrograph of chronically infected and partially lysed cells (olive green) infected with a variant strain of SARS-CoV-2 virus particles (purple), isolated from a patient specimen.  Image credit: NIAIDColorized scanning electron micrograph of chronically infected and partially lysed cells (olive green) infected with a variant strain of SARS-CoV-2 virus particles (purple), isolated from a patient specimen. Image credit: NIAID

About the study

In this study, the researchers obtained data from the Israeli Ministry of Health’s national database during the period between January 10, 2022 and March 2, 2022. They obtained data on vaccination status and demographic variables, such as age, gender, etc., of participants. recruited for this study. The scientists included people aged sixty and over and received four doses of the vaccine (four-dose group). The Omicron variant was the dominant strain circulating in Israel during the study period.

In this study, researchers estimated the incidence of confirmed COVID-19 infection and severe consequences after receiving the fourth dose of vaccine. They further determined the time frame to achieve maximum protection after receiving the fourth dose of the vaccine. The scientists compared the four-dose group with two control groups. The first control group included people who were eligible for a fourth dose but had not yet received it, and this control group was called the three-dose group. The second control group included individuals who had received a fourth dose of vaccine, three to seven days earlier, and were called the internal control group.

Main conclusions

In this study, the researchers reported that compared to three doses of vaccine given at least four months earlier, a fourth dose provided additional short-term protection against confirmed infections and serious illnesses caused by the Omicron variant. In addition, this study found that the incidence rate of confirmed infections was two-fold lower and the frequency of serious outcomes was three-fold lower in people in the fourth week after receiving the fourth dose compared to the three. dose group.

The scientists revealed that maximum protection was observed in the fourth week after vaccination, after which the ratio of rates over time, since the fourth dose, decreased to around 1.1 by the eighth week. This result implies that the degree of protection against a confirmed infection decreases relatively rapidly. In severe disease, the rate ratios associated with the internal control group were lower than the relative rate ratios for the three-dose group. The degree of protection against severe COVID-19 outcomes did not decline at week six after receiving the fourth dose. The authors of this study recommended more follow-ups to determine the extent of protection against serious diseases over a longer period.

Conclusion

The scientists said that although they tried to eliminate as many sources of bias as possible, some sources could have been overlooked. For example, they did not take into account differences in behavior between people who received the fourth dose and those who did not. Additionally, in the setting of severe disease, researchers did not assess the prevalence of comorbidities that may have contributed to disease outcome. Overall, the current study indicated that the fourth dose of the COVID-19 vaccine provided short-term protection against infection confirmed by the Omicron variant. However, scientists have observed that a four-dose vaccination strategy may protect individuals against severe disease better than a three-dose regimen.

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