A new study suggests immunity against Delta infection drops by half within six months, but still prevents severe illness. Heads you win. Tails Covid loses.

(Originally published Oct. 6 in “What in the World“) U.S. government bean counters have determined just how many lives vaccines saved in the first five months of this year. According to the Department of Health and Human Services, vaccinations kept 265,000 people free of infection, 107,000 of those out of the hospital, and 39,000 very lucky individuals out of the clutches of the Grim Reaper.

What’s more interesting is that the DHHS found that for every 10 percentage point increase in inoculations in any given county, the number of hospitalizations and deaths dropped by 11% to 12%. That seems to suggest that vaccines aren’t just improving the surival rates of those who get them, but preventing both infection and transmission. Since that’s exactly what vaccines have been shown to do in clinical studies, these results might seem rather unsurprising. But it’s nice to have confirmation from the field. Unfortunately, the vaccination drive failed to keep more than 700,000 Americans from succumbing to Covid-19; and despite the availability of vaccines, the U.S. death toll this year has already surpassed that in 2020.

With further evidence emerging that the immunity conferred by vaccines ebbs over time, the European Medicines Authority has followed the lead of its constituent governments and approved booster shots for adults six months after receiving their second dose of the Pfizer/BioNTech vaccine. A new study published in The Lancet found that, five months after receiving their second shot, people inoculated with Pfizer/BioNTech enjoy roughly 95% protection against severe illness requiring hospitalization, but only slightly better than 50% protection against infection by the Delta strain of Covid-19.

That means its a coin toss whether anyone past the 5-month mark since receiving their jab will get Delta if they encounter it. So vaccination doesn’t eliminate the numbers game involved in living with Covid, especially when you consider that those with breakthrough cases still get ill and risk suffering “long Covid.”

That’s bad news for countries like the Seychelles, Israel and the UAE that vaccinated the majority of their populations months ago before the global Delta surge had subsided. All three are already rolling out booster shots.

These are first-world problems, of course. Many in the developing world haven’t had a chance to get even a first dose. The World Trade Organization is still warning that lack of access to vaccines is widening the global economic gap. And the International Monetary Fund warned this week that the vaccine divide remains the biggest obstacle to a global economic recovery.