Study: Population Immunity More Important for Controlling Transmission of COVID-19 Than Individual Vaccination Status

Increasing population immunity decreased transmission of COVID-19 and allowed for reductions in quarantine and testing, encouraging 2-dose vaccine and booster uptake.

Maintaining high population immunity, preferably by booster uptake, is essential in determining appropriate quarantine duration for controlling the spread of SARS-CoV-2, according to a study recently published in PNAS Nexus.

The CDC’s reduction of the recommended 7-day quarantine for close contacts of a COVID-19 case to 5 days for the unvaccinated and no quarantine for the vaccinated may not effectively mitigate transmission, according to the authors. Quarantine and testing strategies were found to depend on the incubation period and the transmissibility and level of population immunity against infection.

Researchers compared the probability of post-quarantine transmission and the effective reproduction number with serial testing and isolation for the original SARS-CoV-2 strain and 3 variants of concern, Alpha, Delta, and Omicron, at different levels of vaccination. The comparison of the number of post-quarantine secondary infections between vaccinated and unvaccinated individuals demonstrated that the effect of vaccination status of an individual had only a limited effect on quarantine duration.

Vaccinated cases were less likely to exhibit symptoms than unvaccinated cases, which resulted in less frequent self-isolation. The decreases in case detection and self-isolation increased the probability of releasing an infectious case from quarantine.

Variants of concern produced increased transmission and earlier transmission in the disease time course than the original SARS-CoV-2 strain, challenging the effectiveness of current quarantine and testing strategies. The findings also suggested that determining optimal disease control strategies also depends on the level of population immunity.

Increasing population immunity through vaccine and booster uptake was found to mitigate transmission. Improving 2-dose vaccine uptake led to a substantial decrease in the probability of post-quarantine transmission during serial testing. Increasing uptake of 2-dose vaccines also notably reduced quarantine durations.

However, increasing booster uptake only provided substantive benefit to serial testing compared to quarantine. The decrease in probability of post-quarantine transmission concomitant with an increase in booster uptake was found to diminish as quarantine duration lengthened.

The effectiveness of serial testing in reducing transmission was greatly improved through an increase in booster uptake, according to the study.

Researchers found that scaling up vaccine-acquired immunity from no vaccination to 100% booster intake yielded a reduction in quarantine by 6 days for the original SARS-CoV-2 strain, but only yield a 2-day reduction in quarantine for the Omicron variant. In a population with 100% 2-dose vaccination, scaling up booster intake could justify a reduction of quarantine by as much as 3 days.

However, boosting only has a moderate impact of quarantine duration for Omicron, decreasing it by almost a day. Increasing booster uptake can result in significant differences in the required frequency of serial testing.

Increased 2-dose vaccine uptake also justified reduced surveillance through serial testing, according to the study. Additionally, the findings suggested that rapid antigen testing requires a frequency of every 6 days to curtail transmission within a population that is 100% boosted, encouraging booster uptake.

Overall, an individual’s vaccination status was found to have limited influence on post-quarantine transmission compared to vaccination coverage of the population. Increasing population immunity through vaccination not only decreases transmission, but can also allow for substantial relaxation of requirements for quarantine and testing.

The authors note that surveillance of the immunological status of the population is much more important than the status of the quarantined individual when determining effective quarantine durations. They recommend continued efforts to administer vaccines and booster doses to lessen the burden on health care systems and prevent large outbreaks with emerging variants of concern or their subvariants.


Wells C R, Pandey A, Gokcebel S, et al. Quarantine and serial testing for variants of SARS-CoV-2 with benefits of vaccination and boosting on consequent control of COVID-19. PNAS Nexus. 2022;1(3).