Will an effective COVID-19 vaccine return life to normal in the US?

A new study has shown that the extent to which social distancing restrictions and face mask usage could be relaxed in the United States during the coronavirus disease 2019 (COVID-19) pandemic would greatly depend on how effective a vaccine is and the proportion of the population it covered.

The team - from China, the United States and Australia - conducted a modeling study showing that only a vaccine that was almost 100% effective would suppress the epidemic enough for the US population to return to pre-pandemic life and lose the need for social distancing and face masks.

If a highly effective vaccine could not be obtained, the use of a moderately (80%) effective vaccine in combination with 30 to 40% adoption of face mask usage may be a plausible alternative, say the researchers.

“Vaccination combined with a modest level of non-pharmaceutical measures, such as face mask use in common public spaces (shopping malls and transportation), might be a viable option to continue suppressing the epidemic in the long term,” write Lei Zhang from Xi’an Jiaotong University Health Science Center and colleagues.

The researchers say the study findings could be used to guide plans for the rollout of vaccines and the ongoing implementation of non-pharmaceutical interventions (NPIs).

A pre-print version of the paper is available on the server medRxiv*, while the article undergoes peer review.

Model calibration and data fitting based on reported confirmed COVID-19 cases and deaths in four states, i.e., New York (a-b), Texas (c-d), Florida (e-f), and California (g-h). The blue areas denote 95% confidence intervals. Dashed lines, dash-dot lines, and dotted lines denote the social distancing order (public person-to-person contact rates decreased), face mask order, and reopening (public person-to-person contact rates recovered to no more than 100% of the pre-pandemic level) policies that were implemented in each state, respectively.
Model calibration and data fitting based on reported confirmed COVID-19 cases and deaths in four states, i.e., New York (a-b), Texas (c-d), Florida (e-f), and California (g-h). The blue areas denote 95% confidence intervals. Dashed lines, dash-dot lines, and dotted lines denote the social distancing order (public person-to-person contact rates decreased), face mask order, and reopening (public person-to-person contact rates recovered to no more than 100% of the pre-pandemic level) policies that were implemented in each state, respectively.

The United States has been one of the most severely impacted countries

Since the first cases of COVID-19 were first identified in Wuhan, China, late last year (2019), the unprecedented spread of the pandemic has had devastating effects on global public health and the economy.

The impact in the United States has been particularly severe, with the number of confirmed infections now having reached more than 9.4 million and the number of deaths more than 232,000.

Contour plots of averted infections as a function of vaccine effectiveness and vaccine coverage rate in four states when social distancing is relased to pre-pandemic level shortly after the commencement of vaccination. The first row is maintaining face mask use at baseline levels, the second row to reducing face masks use to half of the baseline level and the third row is no face mask use. The black solid isoclines indicate the threshold that the number of averted infections is zero. The black dashed lines correspond to the minimal vaccine effectiveness and vaccine coverage rate when the number of averted infections is zero.
Contour plots of averted infections as a function of vaccine effectiveness and vaccine coverage rate in four states when social distancing is relased to pre-pandemic level shortly after the commencement of vaccination. The first row is maintaining face mask use at baseline levels, the second row to reducing face masks use to half of the baseline level and the third row is no face mask use. The black solid isoclines indicate the threshold that the number of averted infections is zero. The black dashed lines correspond to the minimal vaccine effectiveness and vaccine coverage rate when the number of averted infections is zero.

Given that a number of candidate vaccines have now entered phase III clinical trials, hopes are growing that social distancing restrictions and the requirement for facemask usage could soon be relaxed so that Americans can start to return to life as they know it.

However, the extent to which these control measures could be eased would depend on the effectiveness of the potential vaccines, which is not currently known.

“To allow careful planning about what restrictions may need to be continued, research is urgently needed to project how the effectiveness of a potential vaccine may affect the trajectory of the COVID-19 pandemic in the US,” say the researchers.

It is also essential to determine how the current NPIs could be incorporated within an overall control strategy that accounts for the varying efficacy of different vaccines, they add.

What did the researchers do?

The team developed dynamic simulation models of COVID-19 transmission for the four most severely affected states in the US, namely New York, Texas, Florida, and California.

The models were designed to account for the differences in social distancing and face mask policies that apply in each state.

The researchers used the models to evaluate the level of vaccine effectiveness and coverage that would be needed to avert COVID-19 cases and deaths in scenarios where social contact was to return to pre-pandemic levels and face mask usage was reduced.

Each of the state-level models was calibrated based on the most recent daily and cumulative COVID-19 data (from January 26th to September 15th, 2020) obtained from the Johns Hopkins University Coronavirus resource center.

What did the models predict?

In the absence of a vaccine, the spread of COVID-19 could be suppressed in the four states by maintaining the current social distancing measures and levels of face mask usage.

However, returning social contact to pre-pandemic levels, without changing the current requirements for face mask use, would result in new outbreaks, say Zhang and colleagues.

This would lead to between 0.8 and 4 million infections and 15,000 to 240,000 deaths across the four states within just one year.

In this scenario, the adoption of vaccination would help to decrease the number of infections and deaths, even if the vaccine effectiveness and coverage were relatively low, say the researchers.

However, if the rate of face mask use fell by 50%, then introducing a weak vaccine (only 50% effective) with low coverage would not be enough to suppress the epidemic.

With the level of face mask usage halved in these states, a weak vaccine would require a population coverage of 55 to 94% to suppress the epidemic, whereas a moderately effective vaccine (80%) would require 32 to 57% coverage and a strong vaccine (100% effective) would only require 24 to 46% coverage.

However, if the use of face masks stopped altogether, a weak vaccine would not be sufficient to suppress the epidemic, even with high coverage, and further major outbreaks would occur.

A moderate vaccine, on the other hand, would suppress the epidemic at a coverage of 48 to 78%, while a strong vaccine would suppress it at a coverage of 33 to 58%.

What do the researchers advise?

“Given that the willingness to take a COVID-19 vaccine in the US has been estimated at only 58%, only a strong vaccine with high effectiveness of nearly 100% would be sufficient to suppress the epidemic alone and enable relaxation of social distancing and face mask requirement,” write Zhang and colleagues.

However, if a strong vaccine is not attainable, a moderately effective vaccine and a face mask usage rate of around 30-40% would be a plausible alternative to achieve this target, they add.

“Findings from this study provide timely information that can be used by policymakers to plan for the potential release of a COVID-19 vaccine and understand its effect across different regions in the US under different social distancing and face mask use scenarios,” concludes the team.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Sally Robertson

Written by

Sally Robertson

Sally first developed an interest in medical communications when she took on the role of Journal Development Editor for BioMed Central (BMC), after having graduated with a degree in biomedical science from Greenwich University.

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