We’ve seen continued high levels of infection around the world in places that also have high rates of vaccination like Chile, the Seychelles and the U.A.E. Not all vaccines are created equal, and lower efficacy ones such as Sinovac’s Coronavac do provide some protection but aren’t ending the pandemic and may not be reducing asymptomatic spread well either.

It’s not surprising then to see continued masking recommendations for those who have been vaccinated from the World Health Organization. Their advice isn’t limited to countries with the most effective vaccines. The CDC disagrees – they are offering advice to the U.S. where mRNA vaccines are predominantly in use.

Here most places have re-opened. There’s an expectation that many businesses will return to the office in the fall. Schools are expected to be open. Already leisure travel has returned to pre-pandemic levels, and in some places has exceeded it. But is this state of affairs going to last?

I’ve been an optimist, guided by science. In June 2020 I wrote that we’d see a vaccine approved before end of year, then “we could have mass vaccinations in spring, then summer 2021 could be normal.” If anything I tempered my expectations too much – I did not anticipate the degree of effectiveness of the mRNA vaccines.

However travel may not be out of the woods yet, for two reasons.

1. As summer ends, and people return to offices, leisure travel may dry up. It traditionally declines in the fall, when airlines rely more heavily on business travel. But while business travel does seem poised to begin a comeback in the fall, it may take time and may not play the supporting role it traditionally dose. This may mean current optimism about travel faces the reality that it isn’t sustainable throughout September, October and up until the Thanksgiving holiday hits.

2. Variants could lead to fear of the virus that keeps people home, or even new restrictions. The U.K. has been fearful of return to normal even though their dominant vaccine performs better than the Chinese vaccines. Israel has seen a rise in cases despite its head start against the world in vaccination with Pfizer-BioNTech.

There were plenty of scary stories about breakthrough infections in Israel, but with their high level of vaccination having a high percentage of vaccinated among positive cases is exactly what you’d expect. And even in the U.K. a resurgence in the virus hasn’t been accompanied by spiking hospitalizations.

The best vaccines continue to perform well against the variants. Where there are breakthrough infections, those haven’t been as severe overall (of course there are anecdotal examples otherwise). And older people tend to have higher levels of vaccination, so cases have clustered amongst younger people who are less vulnerable anyway.

Nonetheless with the Delta variant of the virus we could see another wave of infection, especially as we exit the summer and the benefit of seasonal reduction in transmission.

Notably, the (super rough) math seems to work here. Initial Rt of ~1.1 translated to ~20% of the population infected, which is close to the 18% mathematical expectation. 6/16

— Trevor Bedford (@trvrb) June 30, 2021

Inputting R0 of 1.18 in the final epidemic size equation yields 29% of the susceptible population infected in the Delta wave. 9/16

— Trevor Bedford (@trvrb) June 30, 2021

A back of the envelope estimate is that 62% of the U.S. currently has immunity from vaccination or prior infection, accounting for both the potential of breakthrough infection as well as vaccination to likely be higher among those who haven’t already had the virus. And we’d expect to see another 11% of the population infected by the Delta variant or 36 million people.

This is back of the envelope, and things won’t play out exactly this way. Some unvaccinated people will mix largely with the vaccinated and won’t actually be at risk. Some people without vaccine immunity or prior infection may still be immune. But it’s an order of magnitude significant projection, and a real scenario to consider.

Future infections will skew towards younger people, less susceptible to severe disease. And it will be mostly centered in areas with low levels of immunity such as certain parts of the Southeast. In the U.K. spread is primarily centered amongst the unvaccinated.

The question for travel is two-fold,

1. How will people, including the vaccinated, react to high levels of virus transmission in the community? Will they limit their activity, even if they personally are at low risk thanks to incredibly effective vaccines (though at higher levels of risk than if the virus wasn’t circulating at all, of course)?

2. How will policymakers respond? Will they take the view that anyone not getting vaccinated is basically assuming risk, and that hospitals probably won’t be overwhelmed because the virus will probably circulating amongst younger people most? Or will they react to case numbers and impose restrictions on indoor gathers, re-impose masking requirements, etc.?

Some people may choose not to travel because of a virus resurgence. But if activities for while you’d travel start to close or face a host of restrictions, other people may choose not to travel simply because it’s less enjoyable.

Spread (largely) among the unvaccinated may be a reason policymakers choose to continue the mask mandate past September 13, 2021 even for those who have been vaccinated. Lobbying by airline unions for them to do so is another reason.

We’re not done with the pandemic, in the U.S. or the world, even though those of us who have gotten mRNA vaccines from Moderna or Pfizer-BioNTech continue to appear to have robust protection against variants.