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BACKGROUNDThe BNT162b2 (Pfizer-BioNTech) 2-dose vaccine for children and the BNT162b2 3rd dose for adolescents were approved shortly before the Omicron outbreak in Israel. The effects of these vaccines on the rates of Omicron confirmed infection are not yet clear.METHODSWe extracted data for the Omicron-dominated (sub-lineage BA.1) period December 26, 2021 through January 8, 2022. We compared rates of confirmed Covid-19 infection between children 5-10 years old 14-35 days after receiving the 2nd dose to an internal control group of children 3-7 days after receiving the 1st dose (when the vaccine is not yet effective). Similarly, we compared confirmed infection rates in adolescents 12-15 years old 14-60 days after receiving a booster dose to an internal control group of adolescents 3-7 days after receiving the booster dose. We used Poisson regression, adjusting for age, sex, socioeconomic status, calendar week, and exposure.RESULTSIn the 5-10 age group, the estimated rate of confirmed infection was 2.3 fold (95% CI, 2.0 to 2.5) lower in the 2nd dose group than in the internal control group. In adolescents, the third dose decreased confirmed infection rates by 3.3-fold (95% CI, 2.8 to 4.0).CONCLUSIONSA recent 2-dose BNT162b2 vaccination in children and a recent booster dose in adolescents reduced the rate of confirmed infection compared to the respective internal control groups. Future studies are needed to assess the duration of this protection and protection against other outcomes such as PIMS and long-COVID.

Following a rise in cases due to the delta variant and evidence of waning immunity after 2 doses of the BNT162b2 vaccine, Israel began administering a third BNT162b2 dose (booster) in July 2021. Recent studies showed that the 3rd dose provides a much lower protection against infection with the omicron variant compared to the delta variant and that this protection wanes quickly. In this study, we used data from Israel to estimate the protection of the 3rd dose against severe disease up to 7 months from receiving the booster dose. The analysis shows that protection conferred by the 3rd dose against omicron did not wane over a 7-month period and that a 4th dose further increased protection, with a severe disease rate approximately 3-fold lower than in the 3-dose cohorts.

BACKGROUNDOn January 2, 2022, Israel began administering a fourth dose of BNT162b2 vaccine (Pfizer-BioNTech) to people aged over 60 years and at-risk populations, who had received a third dose of vaccine at least 4 months earlier. The effect of the fourth dose on confirmed coronavirus 2019 disease (Covid-19) and severe illness are still unclear.METHODSWe extracted data for the Omicron-dominated period January 15 through January 27, 2022, from the Israeli Ministry of Health database regarding 1,138,681 persons aged over 60 years and eligible for the fourth dose. We compared the rate of confirmed Covid-19 and severe illness between those who had received a fourth dose at least 12 days earlier, those who had received only three doses, and those 3 to 7 days after receiving the fourth dose. We used Poisson regression after adjusting for possible confounding factors.RESULTSThe rate of confirmed infection was lower in people 12 or more days after their fourth dose than among those who received only three doses and those 3 to 7 days after vaccination by factors of 2.0 (95% confidence interval [CI], 2.0 to 2.1) and 1.9 (95% CI, 1.8 to 2.0), respectively. The rate of severe illness was lower by factors of 4.3 (95% CI, 2.4 to 7.6) and 4.0 (95% CI, 2.2 to 7.5).CONCLUSIONSRates of confirmed Covid-19 and severe illness were lower following a fourth dose compared to only three doses.

Israel began administering a BNT162b2 booster dose to restore protection following the waning of the 2-dose vaccine. Biological studies have shown that a fresh booster leads to increased antibody levels compared to a fresh 2-dose vaccine, which may suggest increased effectiveness. To compare the real-world effectiveness of a fresh booster dose with that of a fresh 2-dose vaccine, we conduct a quasi-experimental study that compares populations that were eligible to receive the vaccine at different times due to age cutoff policies. Our analysis shows that a fresh booster increases protection against confirmed infection by 3.7 (95% CI: 2.7 to 5.2) fold compared to a fresh 2-dose vaccine.

BACKGROUNDInfection with SARS-CoV-2 provides substantial natural immunity against reinfection. Recent studies have shown strong waning of the immunity provided by the BNT162b2 vaccine. The time course of natural and hybrid immunity is unknown.METHODSData on confirmed SARS-CoV-2 infections were extracted from the Israeli Ministry of Health database for the period August to September 2021 regarding all persons previously infected or vaccinated. We compared infection rates as a function of time since the last immunity-conferring event using Poisson regression, adjusting for possible confounding factors.RESULTSConfirmed infection rates increased according to time elapsed since the last immunity-conferring event in all cohorts. For unvaccinated previously infected individuals they increased from 10.5 per 100,000 risk-days for those previously infected 4-6 months ago to 30.2 for those previously infected over a year ago. For individuals receiving a single dose following prior infection they increased from 3.7 per 100,000 person days among those vaccinated in the past two months to 11.6 for those vaccinated over 6 months ago. For vaccinated previously uninfected individuals the rate per 100,000 person days increased from 21.1 for persons vaccinated within the first two months to 88.9 for those vaccinated more than 6 months ago.CONCLUSIONSProtection from reinfection decreases with time since previous infection, but is, nevertheless, higher than that conferred by vaccination with two doses at a similar time since the last immunity-conferring event. A single vaccine dose after infection helps to restore protection.

BACKGROUNDFollowing administration to persons 60+ years of age, the booster vaccination campaign in Israel was gradually expanded to younger age groups who received a second dose >5 months earlier. We study the booster effect on COVID-19 outcomes.METHODSWe extracted data for the period July 30, 2021 to October 6, 2021 from the Israeli Ministry of Health database regarding 4,621,836 persons. We compared confirmed Covid-19 infections, severe illness, and death of those who received a booster ≥12 days earlier (booster group) with a nonbooster group. In a secondary analysis, we compared the rates 3-7 days with ≥12 days after receiving the booster dose. We used Poisson regressions to estimate rate ratios after adjusting for possible confounding factors.RESULTSConfirmed infection rates were ≈10-fold lower in the booster versus nonbooster group (ranging 8.8-17.6 across five age groups) and 4.8-11.2 fold lower in the secondary analysis. Severe illness rates in the primary and secondary analysis were 18.7-fold (95% CI, 15.7-22.4) and 6.5-fold (95% CI, 5.1-8.3) lower for ages 60+, and 22.0-fold (95% CI, 10.3-47.0) and 3.2-fold (95% CI, 1.1-9.6) lower for ages 40-60. For ages 60+, COVID-19 associated death rates were 14.7-fold (95% CI, 9.4-23.1) lower in the primary analysis and 4.8-fold (95% CI, 2.8-8.2) lower in the secondary analysis.CONCLUSIONSAcross all age groups, rates of confirmed infection and severe illness were substantially lower among those who received a booster dose of the BNT162b2 vaccine.

BackgroundOn July 30, 2021, a third (booster) dose of the Pfizer BNT162b2 vaccine was approved in Israel for individuals 60 years or older who had been fully vaccinated (i.e., received two doses) at least five months previously. Here, we estimate the reduction in relative risk for confirmed infection and severe COVID-19 provided by the booster dose.Methods1,144,690 individuals aged 60y and older who were eligible for a booster dose were followed between July 30 and August 22, 2021. We defined dynamic cohorts where individuals initially belong to the ‘non-booster’ cohort, leave it when receiving the booster dose and join the ‘booster’ cohort 12 days later. Rates of infection and severe COVID-19 outcomes per person-days at risk were compared between the cohorts using Poisson regression, adjusting for possible confounding factors.ResultsTwelve days or more after the booster dose we found an 11.4-fold (95% CI: [10.0, 12.9]) decrease in the relative risk of confirmed infection, and a >10-fold decrease in the relative risk of severe illness. Under a conservative sensitivity analysis, we find ≈5-fold protection against confirmed infection.ConclusionsIn conjunction with safety reports, this study demonstrates the effectiveness of a third vaccine dose in both reducing transmission and severe disease and indicates the great potential of curtailing the Delta variant resurgence by administering booster shots.

BackgroundStarting December 2020, Israel began a mass vaccination campaign against coronavirus administering the Pfizer BNT162b2 vaccine, which led to a sharp curtailing of the outbreak. After a period with almost no SARS-CoV-2 infections, a resurgent COVID-19 outbreak initiated mid June 2021. Possible reasons for the breakthrough were reduced vaccine effectiveness against the Delta variant, and waning immunity. The aim of this study was to quantify the extent of waning immunity using Israel’s national-database.MethodsData on all PCR positive test results between July 11-31, 2021 of Israeli residents who became fully vaccinated before June 2021 were used in this analysis. Infection rates and severe COVID-19 outcomes were compared between individuals who were vaccinated in different time periods using a Poisson regression, stratifying by age group and adjusting for possible confounding factors.ResultsThe rates of both documented SARS-CoV-2 infections and severe COVID-19 exhibit a statistically significant increase as time from second vaccine dose elapsed. Elderly individuals (60+) who received their second dose in March 2021 were 1.6 (CI: [1.3, 2]) times more protected against infection and 1.7 (CI: [1.0, 2.7]) times more protected against severe COVID-19 compared to those who received their second dose in January 2021. Similar results were found for different age groups.ConclusionsThese results indicate a strong effect of waning immunity in all age groups after six months. Quantifying the effect of waning immunity on vaccine effectiveness is critical for policy makers worldwide facing the dilemma of administering booster vaccinations.

Worldwide shortage of vaccination against SARS-CoV-2 infection while the pandemic is still uncontrolled leads many states to the dilemma whether or not to vaccinate previously infected persons. Understanding the level of protection of previous infection compared to that of vaccination is critical for policy making. We analyze an updated individual-level database of the entire population of Israel to assess the protection efficacy of both prior infection and vaccination in preventing subsequent SARS-CoV-2 infection, hospitalization with COVID-19, severe disease, and death due to COVID-19. Vaccination was highly effective with overall estimated efficacy for documented infection of 92{middle dot}8% (CI:[92{middle dot}6, 93{middle dot}0]); hospitalization 94{middle dot}2% (CI:[93{middle dot}6, 94{middle dot}7]); severe illness 94{middle dot}4% (CI:[93{middle dot}6, 95{middle dot}0]); and death 93{middle dot}7% (CI:[92{middle dot}5, 94{middle dot}7]). Similarly, the overall estimated level of protection from prior SARS-CoV-2 infection for documented infection is 94{middle dot}8% (CI:[94{middle dot}4, 95{middle dot}1]); hospitalization 94{middle dot}1% (CI:[91{middle dot}9, 95{middle dot}7]); and severe illness 96{middle dot}4% (CI:[92{middle dot}5, 98{middle dot}3]). Our results question the need to vaccinate previously-infected individuals.

SummaryBackgroundOne of the significant unanswered questions about COVID-19 epidemiology relates to the role of children in transmission. In this study we estimate susceptibility and infectivity of children compared to those of adults. Understanding the age-structured transmission dynamics of the outbreak provides precious and timely information to guide epidemic modelling and public health policy.MethodsData were collected from households in the city of Bnei Brak, Israel, in which all household members were tested for COVID-19 using PCR. To estimate relative transmission parameters in the absence of data on who infected whom, we developed an estimation method based on a discrete stochastic dynamic model of the spread of the epidemic within a household. The model describes the propagation of the disease between household members allowing for susceptibility and infectivity parameters to vary among two age groups. The parameter estimates are obtained by a maximum likelihood method, where the likelihood function is computed based on the stochastic model via simulations.FindingsInspection of the data reveals that children are less likely to become infected compared to adults (25% of children infected over all households, 44% of adults infected over all households, excluding index cases), and the chances of becoming infected increases with age. An interesting exception is that infants up to age one year are more likely to be infected than children between one and four. Using our modelling approach, we estimate that the susceptibility of children (under 20 years old) is 45% [40%, 55%] of the susceptibility of adults. The infectivity of children was estimated to be 85% [65%, 110%] relative to that of adults.InterpretationIt is widely observed that the percentage of children within confirmed cases is low. A common explanation is that children who are infected are less likely to develop symptoms than adults, and thus are less likely to be tested. We estimate that children are less susceptible to infection, which is an additional factor explaining their relatively low rate of occurrence within confirmed cases. Moreover, our results indicate that children, when infected, are somewhat less prone to infect others compared to adults; however, this result is not statistically significant.The resulting estimates of susceptibility and infectivity of children compared to adults are crucial for deciding on appropriate interventions, and for controlling the epidemic outbreak and its progress. These estimates can guide age-dependent public health policy such as school closure and opening. However, while our estimates of children’s susceptibility and infectivity are lower than those of adults within a household, it is important to bear in mind that their role in the spread of COVID-19 outside the household, is also affected by different contact patterns and hygiene habits.