With a limited supply of Covid-19 vaccines, the question on many minds is whether to dilute the vaccines or delay the second dose to speed Covid-19 vaccination. My answer is neither—the greatest threat to population protection is virus resistance. Diluting the vaccine or delaying the second dose is a reckless wager, which may give SARS-CoV-2 an opportunity to mutate beyond the power and protection of our current set of vaccines. Yet, despite acknowledging the risk, countries like the UK still seem intent on pursuing the option, putting each and every one of us at risk.
All the vaccines currently approved require two doses administered three to four weeks apart. The first shot starts building protection and the second shot helps elevate the levels of protection the vaccine has to offer. Data on protection from a single dose of the vaccine is sparse, but we know that the concentration of protective antibodies is low after one dose of the vaccine or after two doses of diluted vaccines.
For the Pfizer vaccine, for example, a study published in the BMJ found that the vaccine was only 52% effective after the first dose. Only after the second dose did protective levels rise to 95%. There are reports that a single dose of the Moderna vaccine may be more protective, up to 80%, but the truth is that no one can say with certainty how protective a single dose may be and for how long. We do know, however, that most routine vaccinations require multiple doses over months and sometimes years to be fully protective. We should not assume that the Covid vaccine will be any different.
Those suggesting that two diluted doses of a vaccine would provide the same protection as a two full doses are working off of even less solid evidence to back their claims. There is no data publicly available on how effective these diluted vaccines may be. Even executives at the pharmaceutical companies who have a dog in the fight have hesitated to sponsor the approach and the FDA has called it “premature and not rooted solidly in the available evidence”.
Despite the lack of data, we can say with near certainty that a diluted or delayed vaccine will lead to a large population with low antibody titers, which is the ideal condition for creating vaccine resistance. New strains of SARS-CoV-2 already exist that are more transmissible and lack at least two essential sites for virus neutralization. Contrary to many public statements, SARS-CoV-2 can and does mutate to evade immune pressure and can do so in a single individual. There is no telling what the implications may be of only partially vaccinating a large swath of the population against a virus as clever and wily as this one.
Vaccine production is ramping up rapidly in the US, Europe, Russia, China, and India. Strategies intended to provide minimal protection to many place everyone, vaccinated or not, at needless risk from SARS-CoV-2 variants that no longer respond to extant vaccines.
Rather than shortening the pandemic, delay and dilute strategies will most likely prolong the pandemic indefinitely.