Second Covid-19 vaccine timing and 3 other critical questions, answered

At last, the moment you had been waiting for arrived: You were eligible to get the Covid-19 vaccine. You made an appointment, waited in line, rolled up your sleeve, and got the shot. If you got one of the mRNA vaccines, Pfizer, or Moderna, you also agreed to come back in three to four weeks for the second dose. But then... you didn’t.

Maybe you were freaked out by stories of second-shot side effects. Maybe you had an emergency the day you were supposed to go back. Or maybe you just forgot. Whatever the reason, all is not lost if you missed your second dose appointment. (But you do need to go back if you want the full 94 to 95 percent protection the mRNA vaccines offer.)

George Rutherford, Director of the Prevention and Public Health Group and professor of epidemiology and biostatistics at the University of California San Francisco, explains to Inverse why that is.

He offers guidance on four emerging scenarios now that it’s easier than ever to get a Covid-19 vaccine in the United States.

• What to do if you only got one Moderna or Pfizer Covid-19 shot
• What to do if you missed your window for the second dose
• What to do if you forget your vaccination card
• Can you mix and match Covid-19 vaccines?

Here’s everything you need to know.

4. What if you only got one Covid-19 shot?

With new variants popping up at a rapid clip, it’s more important than ever to get that second shot. A study published in Science in April found that people who hadn’t previously had Covid-19 and only got one shot of Pfizer didn’t produce detectable levels of antibodies against certain variants.

Rutherford’s answer, in turn, is simple and direct: get the second one. “Unless we’re talking about Johnson and Johnson, you should get the second one and you should get the same [brand] you got before,” he tells Inverse.

If you got Moderna for the first shot, that’s what you should get for the second shot. If you got Pfizer, you should get Pfizer again.

3. What if you missed your window for the second Covid shot?

Rutherford says the actual window is longer than you think. “The three to a four-week window is the minimum amount of time,” he says.

When the public health officials were trying to figure out what the recommended window should be, they had to reach a compromise between the body producing a robust immunological response and getting people vaccinated as quickly as possible, Rutherford explains.

“Obviously I don’t mean waiting five years or something,” he’s quick to add. “But Canada has changed their window to four months. I think four to five months is fine.”

So why does the Centers for Disease Control recommend no more than 42 days between doses?

That’s when the efficacy rate — the percentage reduction of disease in a vaccinated group — of the Pizer and Moderna vaccines is very high. If the efficacy is at this 94 to 95 percent rate, then we’re more likely to quash potential new variants and get the pandemic under control.

Canada changed its window out of desperation: They’re dealing with a devastating outbreak and want to get as many people as possible at least one dose of the vaccine.

There is also evidence that the AstraZeneca vaccine, which is not currently approved in the United States, is more effective within a three-month window than its original six-week window.

The bottom line is this: If you missed the second dose, get it as soon as you can. Given that Americans under 65 years old became eligible in March/April — and if you’re in that age group — you’re still in the window for your second shot.

If you missed your appointment for the second dose, you can make another one. It is not required to return to the same location where you received your first shot. Even if your second dose is significantly delayed, you don’t need to start the vaccine series from the start. Contact your vaccine distributor and reschedule as soon as possible.

2. What if you forget your Covid-19 vaccination card?

“They’ll make a new one up for you,” Rutherford says. Vaccination rates get reported to local and state health departments, so it’s very likely they have a record of when you were first vaccinated.

Of course, you can always just leave and come back with your card a few days later. Remember, public health officials want you to get vaccinated, they’re not trying to make the process harder than is absolutely necessary.

1. Can you mix Covid-19 vaccines?

While there have been reports of people accidentally getting the wrong second shot, Rutherford says you should avoid it.

“The CDC put out a statement saying it’s ok to mix Pfizer and Moderna in exceptional circumstances,” he explains, “but you should avoid it if possible.”

Preliminary tests suggest there may be some benefit to mixing AstraZeneca and the Pfizer vaccine, but Rutherford cautions that we need much more data before recommending anything of the kind. Not only is AstraZeneca not approved yet in the United States, but they’re two different kinds of technology, he explains.

“AstraZeneca is made from a weakened version of a live virus and mRNA vaccines don’t use any virus at all,” he explains.

The preliminary testing that was done mixing Pfizer and AstraZeneca used a relatively limited group, 830 volunteers all over the age of 50. There’s much more work to be done before drawing any broad conclusions about the safety or efficacy of mixing two different types of vaccines.

Being fully vaccinated is incredibly important for your health and public health. But we get it, plans go wrong. What matters is there’s still time to make it right.