Social distancing, other infection controls still needed to achieve herd immunity

Vaccinating those most vulnerable to Covid-19 will not be sufficient to reach herd immunity if social distancing, mask wearing, and other non-pharmaceutical interventions to limit viral spread are lifted too soon, according to findings from a mathematical modeling study from the United Kingdom.

The study is just the latest to warn against the early relaxation of such interventions, even as vaccinations ramp up and more people receive their shots.

The modeling predicted that even under optimistic assumptions of vaccine efficacy, removal of all non-pharmaceutical interventions once the vaccination program is complete in the UK will lead to 21,400 avoidable deaths from Covid-19.

Under less optimistic vaccine efficacy estimates, early removal of infection-control strategies was predicted to lead to more than 95,000 avoidable deaths.

“Early relaxation of non-pharmaceutical interventions (NPIs), before sufficient immunity has been established, would precipitate a large wave of infection, with resultant hospital admissions and deaths,” wrote researcher Sam Moore,PhD of the University of Warwick, Coventry, UK, and colleagues in The Lancet Infectious Disease. “A similar effect is predicted from any final release of NPIs if the herd immunity threshold has not been achieved.”

The researchers noted that even with high levels of vaccine uptake, a substantial portion of the population will need to be immunized to prevent subsequent infection waves, “implying that strong NPIs would still be required even when phase 1 of the vaccination program (offering vaccine to all people older than 50 years) is complete to avoid surges in infection.”

“A more gradual measured approach, in which NPIs are gradually released over a period of many months, has advantages over sudden changes to controls but still might not mitigate the worst effects,” they added.

The mathematical model constructed by Moore and colleagues was designed to predict the long-term spread of SARS-CoV-2 under the planned vaccine rollout in the UK using epidemiological data and varying estimates of vaccine efficacy.

That planned rollout involved a two-dose vaccination program, with doses given 12 weeks apart, and protection onset projected at 14 days after vaccination.

The model assumed default vaccine uptake (default uptake scenario) of 95% in those aged 80 years and older, 85% in those aged 50–79 years, and 75% in those aged 18–49 years, and then varied uptake optimistically and pessimistically.

Vaccine efficacy against symptomatic disease was assumed to be 88% on the basis of Pfizer-BioNTech and Oxford-AstraZeneca vaccines being administered in the UK, and protection against infection varied from 0% to 85%.

The researchers considered the combined interaction of the UK vaccination program with multiple potential future relaxations (or removals) of NPIs, to predict the reproduction number (R) and pattern of daily deaths and hospital admissions due to Covid-19 from January, 2021 to January, 2024.

Based on the modeling, the researchers concluded that vaccination of vulnerable populations alone would not be sufficient to contain the Covid-19 outbreak.

“In the absence of NPIs, even with our most optimistic assumption that the vaccine will prevent 85% of infections, we estimate R to be 1.58 (95% credible intervals [CI] 1.36–1.84) once all eligible adults have been offered both doses of the vaccine,” Moore and colleagues wrote.

Under the default uptake scenario, removal of all NPIs once the vaccination program is complete was predicted to lead to 21,400 deaths (95% CI, 1,400–55,100) from Covid-19 for a vaccine that prevents 85% of infections. This number increased to 96,700 predicted deaths (95% CI, 51,800–173,200) if the vaccine only prevents 60% of infections.

The researchers wrote that while vaccination substantially reduces total deaths, it only provides partial protection at the individual recipient level.

They estimated that for the default uptake scenario at a vaccine protection of 60% against infection, 48.3% (95% CI, 48.1–48.5%) and 16.0% (15.7–16.3%) of deaths will occur among people who have received one or two doses of the vaccine, respectively.

In commentary published with the study, PhD candidate Sebastian Contreras and physicist Viola Priesemann of the Max Planck Institute, Göttingen, Germany, noted that the researchers did not explicitly include the impact of variants that may escape immune response in their modeling.

“Escape variants might have a devastating effect; at worst, they could force us to start the vaccination program from scratch, including the necessity to re-enforce strong restriction,” they wrote. “If escape variants surge during a new wave of infections, they could hit already overburdened health-care systems, and there will be little capacity to react.”

They added that avoiding such a wave is critical to mitigating the impact of potential escape variants, and they proposed lifting restrictions “at a pace with vaccination” to allow for more contact while reducing the risk of another surge in infections.

“In every country, we have to decide how to use the protection of vaccines wisely to prevent further waves of SARS-CoV-2,” they wrote. “Waves that hit those who have not been offered a vaccine will spread to unprotected people and unprotected countries, which could lead to further evolution of escape variants.”

  1. Vaccinating those most vulnerable to Covid-19 will be insufficient to reach herd immunity if social distancing, mask wearing, and other non-pharmaceutical interventions to limit viral spread are lifted too soon, mathematical modeling suggests.
  2. Even under optimistic assumptions of vaccine efficacy, removal of all non-pharmaceutical interventions once the vaccination program is complete in the UK was projected to lead to 21,400 avoidable deaths from Covid-19.

Salynn Boyles, Contributing Writer, BreakingMED™

The study was funded by the National Institute for Health Research, Medical Research Council, and UK Research and Innovation.

Moore reported grants from the National Institute for Health Research outside the submitted work.

Contreras and Priesemann disclosed no relevant relationships.

 

Cat ID: 926

Topic ID: 79,926,933,926,927,928,934